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uNivERsmry 

PENNSYLVANIA. 
UBRAR1E5 




ALKOXYSILANES CONSOLIDATION OF STONE 
AND EARTHEN BUILDING MATERIALS 

Isil Oztiirk 

A THESIS 

in 

The Graduate Program in Historic Preservation 

Presented to the faculties of the University of Pennsylvania in 
Partial Fulfillment of the Requirements for the Degree of 

MASTER OF SCIENCE 



1992 



/HaxuL l^cohsT^iCO 



Jeyanne Marie Teutonico, Lecturer, Historic Preservation 
idvisor 



\v- 



--.^ank G. l^Iatero, Associate Professor of Architecture in Historic Preservation 
R^adet_. 





Javid G. De LohgTProfessor of Architectb 
Graduate Group Chairman 



EARTS 



UNIVERSITY 

OF 

PENNSYLVANIA 

LIBRARIES 



TABLE OF CONTENTS 

CHAPTER 1: OVERVEW OF STONE CONSOLIDATION MATERIALS 1 

LI. Introduction 1 

1.2. Inorganic Stone Consolidants 3 

1.2.1. Siliceous Consolidants 4 

1.2.1.1. Alkali Silicates 4 

1.2.1.2. Fluorosilicon Compounds 5 

1.2.2. Alkaline Earth Hydroxides 6 

1.2.2.1. Calcium Hydroxide (Limewater) 6 

1.2.2.2. Barium Hydroxide 8 

1.3. Alkoxysilanes 10 

1.4. Synthetic Organic Polymers 12 

1.4.1. Acrylic Polymers 13 

1.4.2. Acrylic Copolymers 14 

1.4.3. Epoxies 16 

1.5. Waxes 18 

CHAPTER 2: ALKOXYSILANES 26 

2.1. Performance Criteria 26 

2.1.1. Performance Criteria for Stone Consolidants 26 

2.1.2. How Alkoxysilanes Meet the Performance Criteria 29 

2.2. Chemistry of Alkoxysilanes 30 

2.2.1. Chemical Structure of Alkoxysilanes 31 

2.2.2. Polymerization of Alkoxysilanes: Hydrolysis and Condensation 33 

ii 



2.3. Application Methods 39 

2.4. Protection and Maintenance 41 

2.5. Evaluation Techniques 42 

2.5.1. Laboratory Testing Program 43 

2.5.2. Field Testing Program 47 

CHAPTER 3: REVffiW OF USE OF ALKOXYSILANES 

IN CONSERVATION 54 

3.1. Consolidation of Stone 54 

3.1.1. History of the Use of Alkoxysilanes for Stone Consolidation 54 

3.1.2. Recent Experiments with Alkoxysilanes in Stone Consolidation 56 

3.2. Consolidation of Earthen Building Materials 65 

CHAPTER 4: CONCLUSIONS AND RECOMMENDATIONS 77 

APPENDIX A: COMMERCIALLY AVAILABLE ALKOXYSILANE 

CONSOLIDANTS 80 

APPENDIX B: TEST METHODS FOR THE LABORATORY 

EVALUATION OF STONE 84 

APPENDK C: SELECTED PROJECTS TREATED WITH ALKOXYSILANES ... 87 
APPENDIX D: MEETINGS ON THE CONSERVATION OF 

EARTHEN ARCHITECTURE 94 

BIBLIOGRAPHY 95 



m 



CHAPTER 1 
OVERVIEW OF STONE CONSOLIDATION MATERIALS 

1.1. INTRODUCTION: 

The increasing interest in the conservation of historic structures and the accelerating 
deterioration of the stone of exposed sculptures and buildings have necessitated an 
understanding of the mechanisms responsible for stone decay to develop the possibilities of 
optimum stone protection. In response to this, numerous research projects, publications 
and conferences dealing with stone have been facilitating the worid wide exchange of 
information. 

As is the case with other materials, stone experiences change upon exposure to 
natural weathering. As long as stone is in contact with any kind of environment, it 
undergoes chemical, mechanical, physical or biological weathering processes. Weathering 
is the natural disintegration and erosion of stone caused by the action of water, wind, and 
atmospheric gases.' When used in building construction or for outdoor sculpture, stone is 
subject to other decay processes in addition to natural weathering due to its interaction with 
polluted urban-industrial environments, to improper selection or positioning of stone 
material, etc. 

The factors considered responsible for the deterioration of exposed stone include 
salt crystallization, chemical attack by acidic substances which are either naturally occuring 
constituents of the atmosphere or introduced by industrial and automotive combustion, 
freezing of water in pores and capillaries in the stone, microbiological growth on stone 
such as bacteria, algae and fungi, repeated wetting and drying of stone, thermal stresses 
caused by differential thermal expansion of some mineral constituents, and abrasion due to 

1 



wind or windbome particles or human contact. 

Water is the most aggressive agent which acts as a vehicle for weathering 
processes. Water dissolves and transports soluble salts within the stone causing 
efflorescence on the surface and salt-induced spalling. The combination of water with 
gaseous pollutants results in acidic precipitation. Water is responsible for frost damage in 
climates where freezing temperatures can occur. It is also water which favours the growth 
of microorganisms. Disintegration, surface erosion, cracking and crust formation are the 
commonly observed symptoms of stone decay. 

As a consequence of the combined effects of chemical and mechanical weathering, 
stone can lose its cohesion to such a degree that its physical survival is imperilled and a 
treatment is necessary to restore its integrity. In such cases, consolidation could become 
part of the conservation process. Consolidation is an in-depth treatment that re-establishes 
the cohesion between particles of deteriorated stone.^ The consolidant is usually applied as 
a liquid which is intended to penetrate deeply into the stone and deposit an additional 
binding agent which will reinstate the stone's cohesion. Besides its consolidating value, a 
consolidant should meet basic performance requirements concerning compatibility with 
stone, effect on moisture transfer, effect on stone porosity, durability, depth of penetration, 
and effect on appearance. These factors will be discussed in Chapter 2. 

Consolidation should not be considered as a single operation. It is a part of a series 
of processes which include diagnosis, cleaning, preconsolidation, consolidation, surface 
protection, and maintenance.^ Additionally, consolidation should be performed only in 
specific cases when the degree of deterioration threatens the integrity of the material and 
after considering other less invasive treatment options. 

The stone consolidants reviewed in this chapter are divided into four groups 
according to their chemistry. They are inorganic consolidants, alkoxysilanes, synthetic 
organic polymers , and waxes. 

2 



1.2. INORGANIC STONE CONSOLIDANTS: 

Inorganic materials, such as soluble glass, waterglass, limewater, barium 
hydroxide, etc., have been used as stone consolidants since the 19th century. Despite more 
than a hundred years of experience in the application of various chemical substances to 
consolidate stone,^ little success has been achieved with inorganic materials. Although 
well-intended, their use often accelerated the deterioration process rather than reducing it.^ 
There is still not much known about efficient inorganic substances or feasible application 
methods. Recent improvements in the application of some inorganic consolidants such as 
calcium hydroxide^ and barium hydroxide^ have renewed interest in inorganic consolidants. 

The use of inorganic materials as consolidants is an effort to produce a decay- 
resistant phase in the pores of deteriorated stone from aqueous solutions. The deposition, 
that will bind stone particles together, can result from evaporation of the solvent or 
chemical reactions with the stone. It has been assumed that the binding agent of the 
consolidant should be similar in nature to the original cement of the stone. For example, it 
is generally agreed that calcareous stones such as limestone and marble should be 
consolidated with consolidants which result in the formation of calcium or barium 
carbonate; sandstones, on the other hand, should be consolidated with consolidants which 
deposit silica as an end product.* 

Despite their ability to structurally regenerate stone and remain stable over time, 
inorganic consolidants have failed to meet many of the performance requirements of a 
consolidant. In general, their drawbacks include insufficient penetration and formation of 
shallow and hard surface crusts, formation of soluble salts as by-products in the 
consolidation reaction, the growth of precipitated crystals, inability to improve the 
mechanical properties of stone, and possible change in the color of stone. 

Siliceous consolidants and alkaline earth hydroxides are the primary inorganic 

3 



materials used in stone consolidation. Other inorganic materials, including hydrofluoric 
acid, aluminum sulfate, zinc and aluminum stearates, phosphoric acid and phosphates have 
been used to a lesser extent to consolidate stone.' 

1.2.1. SILICEOUS CONSOLIDANTS: 

Siliceous consolidants have been used to consolidate sandstone and limestone based 
up>on the principle that formation of silica or insoluble silicates occurs within the pores of 
the stone to give the grains of the stone a protective coating and bind them together. 

Silica (SiOj) is an inorganic compound of the chemical element silicon (Si). 
Hydrated silicon oxides, which is the combination of silica and water, form salts called 
silicates, such as sodium silicate and potassium silicate. The reaction of those silicates with 
water, which is called hydrolysis, results in the formation of silicic acid (Si(OH)4) and a 
caustic by-product, sodium or potassium hydroxide, better known as caustic soda or 
caustic potash. It is this silicic acid that is considered responsible for the consolidation of 
stone. *° The silicic acid condenses rapidly precipitating silica gel in the stone pores under 
the influence of acidic oxides from the air, such as COj, SO2, NO2, etc." If silicic acid is 
formed inside a stone which carries hydroxyl (-0-H) groups on its surface, a reaction takes 
place between the acid and the stone in which water is released. As a result of that, 
chemical bonds are formed, improving cohesion of the stone. '^ 

Siliceous consolidants include alkali silicates and fluorosilicon compounds. 

1.2.1.1. ALKALI SILICATES: 

Alkali silicates have been known since about 1500 B.C.'^ Around 1830, the 
chemist Von Fuchs began to manufacture sodium silicate or soluble glass (Na2Si03). 
Initially, the consolidation of stone with soluble glass had a big success, but then met 
serious criticism and was progressively abandoned, i"* Soluble glass was used around 1857 

4 



to treat the stone of Westminster Abbey in England, however there is no information 
available about the long-term effectiveness of this treatment*^ 

On the other hand, a closely related chemical potassium silicate (KjSOj), known as 
"waterglass" has been marketed for stone preservation to the present day. Although some 
success was reported in the use of waterglass in Germany,'^ in general alkali silicates are 
not recommended for stone consolidation. (It is essential to note that the term "waterglass" 
is a colloquial term used to refer to potassium and/or sodium silicates.) i'' 

One of the drawbacks of alkali silicates is the formation of water soluble salts, such 
as sodium carbonate and potassium carbonate, as a result of carbonation. These salts can be 
deleterious to the stone by migrating and forming efflorescence on the stone surface, or 
causing typical subflorescence damage, e.g. 

KjSiOj.aq + CO2 — > SiOj.aq + KjCOj 

(water solution of (atmospheric (silica gel) (potassium carbonate) 
potassium silicate) carbon dioxide) 

Alkali silicates also produce impervious surface crusts, since silicates precipitate 
relatively rapidly and they are deposited near the surface of the treated stone. Formation of 
silica gel at the surface prevents the further absorption of atmospheric carbon dioxide and 
therefore prevents the complete and deep carbonation of the silicated solution. In addition, 
their high viscosity is also responsible for insufficient penetration. 

Despite the poor performance of alkali silicates, these types of stone consolidants 
have never completely disappeared from the market. Some recent attempts at improving 
waterglass appUcation have been made, but unsatisfactory results were obtained.'* 

1.2.1.2. FLUOROSILICON COMPOUNDS: 

In 1883, L. Kessler suggested the use of metal salts of fluorosilicic acid as an 



alternative method to the use of waterglass.^' Ruorosilicates react with calcium carbonate 
and form silica gel and insoluble fluorides which have consolidating and waterproofing 
properties, e.g. 

MgSiFg + 2CaC03 — > ICaFj + MgFj + 2CO2 + SiOj 
(magnesium (calcium (calcium (magnesium (silica gel) 

fluorosilicate) carbonate) fluoride) fluoride) 

Many soluble types of fluorosilicates, such as magnesium, zinc, and aluminum 
fluorosilicates have been used to consolidate limestone and sandstone. In Italy, a mixture of 
magnesium and zinc fluorosilicates has been used for the restoration of several stone 
monuments by P. Sampaolesi and some success has been achieved.^o 

However, similar to the alkali silicates, only a surface hardening of the stone is 
achieved by the use of fluorosilicates which may lead to future spalling. In addition, a 
tendency to discolor the stone and to form soluble salts which damage the stone through 
salt crystallization are other undesirable effects of the fluorosilicates. Deposition of 
magnesium fluorosilicate salts forms a whitish color on the treated stone. 

A systematic study of several stone treatments, carried out by B. Penkala in 1964, 
indicated that fluorosilicates were not effective consolidants.^' In rrwst countries,the use of 
these consolidants in conservation is not allowed.22 

1.2.2. ALKALINE EARTH HYDROXIDES: 

Alkaline earth hydroxides include calcium hydroxide and barium hydroxide. 
L2.2.1. CALCIUM HYDROXIDE (LIMEWATER): 

Limewater is a clear saturated calcium hydroxide solution. It has been used for 
centuries to treat limestone. When in solution or wetted, calcium hydroxide reacts with 
carbon dioxide in the air and forms insoluble calcium carbonate which is chemically 
identical to limestone. The precipitated calcium carbonate in stone may bind detached 

6 



limestone particles together. It is, therefore, believed that limestone consolidated with 
limewater retains its original properties and behavior. 

Repeated applications of limewater are necessary to produce sufficient calcium 
carbonate for consolidation, due to the low water solubility of calcium hydroxide.^^ In 
addition, the calcium hydroxide solution should be fairly dilute to prevent the formation of 
calcium carbonate on the surface of stone and allow deeper penetration.^'* The precipitation 
of calcium carbonate begins at the surface as it is dependent on the absorption of carbon 
dioxide from the air. 

Recent improvements in consolidating the mediaeval figure sculptures of Wells 
Cathedral, England, with lime poultices and limewater have attracted new attention to 
limewater as a stone consolidanL In the early 1970's, Prof. Robert Baker developed a lime- 
based technique for the conservation of the west front of Wells Cathedral, known as the 
"lime method", or the "Baker" or "Wells " method.^ The lime method is mainly a series 
of procedures, which are in sequence: cleaning, consolidation, repair, and surface coating. 
The application of a hot lime poultice to the pre-wetted Doulting limestone for cleaning is 
followed by repeated applications of limewater (almost forty applications), flooded onto the 
siuface over a period of days for consolidation. When necessary, lime mortar is used for 
surface repairs. Finally, all the cleaned and repaired stone is given a shelter coat consisting 
of lime, sand and stone dust to provide some protection after treatment. 

According to some advocates of the process, the lime poultice alone has a noticeable 
effect in consolidating the stone.^ It has been claimed that the lime poultice makes the 
surface more receptive to the lime water applications and also achieves some initial 
strengthening of friable areas.^'' However, most practitioners agree that the limewater 
applications rather than the lime poultice play the major part in consolidation. 

Although no further signs of decay have been reported after the treatment at Wells, 
the subsequent experiment carried out by C. Price and K. Ross on the same weathered 

7 



stone in laboratory conditions replicating the Wells method did not provide any conclusive 
evidence of consolidation either with the poultice or the limewater.^^ In 1988, another 
experiment carried out by Price et al,^ using a radioactive tracer to monitor the deposition 
of lime, demonstrated that more than half of the lime deposited is in the outer two 
millimeters of the stone. The radioactive tracer has the advantage of differentiating between 
the calcium already present in the stone and calcium deposited from the calcium hydroxide 
solution. Further results of this experiment showed that the limewater deposits four to five 
times as much lime as the lime poultice, although there was evidence that the poultice may 
facilitate the efficacy of the limewater treatment 

Despite its long history of use as stone consolidant, the effectiveness of the lime 
water technique is, thus, still open to debate. The question remains whether it is the skill of 
the conservator or the chemical properties of limewater, the lime poultice, or treatment as a 
whole which accounts for the success of the lime technique in certain cases. 

1.2.2.2. BARIUM HYDROXIDE: 

Barium hydroxide [Ba(OH)2], formerly termed baryta, solution has been used in 
place of calcium hydroxide solution for the purpose of hardening and protecting weathered 
stone since the 19th century. In 1862 Prof. A. H. Church obtained a patent for the use of 
barium hydroxide solution in stone preservation.^ Similar to calcium hydroxide, barium 
hydroxide reacts with carbon dioxide resulting in the precipitation of insoluble carbonates. 
However, the treatment with barium hydroxide also somewhat transforms soluble calcium 
sulfate, which is present in the stone after years of attack by atmospheric sulfur oxides, to 
much less soluble barium sulfate.^ • Barium sulfate is more resistant to atmospheric 
erosion.32 Thus, stone treated with barium hydroxide may be protected against further 
weathering. 

Although initially no failure in the use of barium hydroxide solution on calcareous 

8 



stone was reported by Church, the treatment later met considerable criticism from other 
conservators.^^ Most importantly, it has been claimed that no marked hardening effect is 
produced with this technique, since Church's method allows the barium hydroxide solution 
to dry out shortly after application, resulting in the deposition of a very fine, non-adherent 
powder of barium carbonate.^'' According to A.R.Wames and J.E. Marsh, crystalline 
inorganic precipitates, such as barium carbonate and barium sulfate, do not have a long- 
term consolidating effect.^^ With the use of barium hydroxide solution, only a surface 
hardening is produced and eventually this dense surface layer exfoliates. The crystal 
growth of barium carbonate or barium sulfate can also cause exfoliation, because these 
crystals have a larger molecular volume than the calcite crystals of calcareous stone and 
deteriorated stone might not accomodate these relatively large crystals. Additionally, this 
technique has been found to alter the surface color or texture of stone in many cases.^*^ 

Despite such negative judgements, in 1966, S. Z. Lewin re-proposed the use of 
barium hydroxide solution in a revised form for stone treatment'' Lewin's initial proposal 
includes the use of an homogenous solution of barium hydroxide and urea. This procedure 
was proposed only for use by immersion. In 1971, Lewin made modifications to the 
process by adding glycerine to the barium hydroxide-urea solution which would allow in 
situ application.'* The addition of urea and glycerine to the barium hydroxide solution 
fundamentally differentiates this new method from the previous technique. The aim of this 
revised method is to achieve the deposition of barium carbonate as a binder deeply within 
the pores of the stone by keeping the barium hydroxide solution in contact with the stone 
for a prolonged period, usually 2 to 3 weeks. The glycerine prevents the solution from 
drying out for that period, while the urea facilitates deep penetration of the hydroxide and 
constitutes a source of carbon dioxide through its hydrolysis in the solution.'' 

The process occurs in two phases.^ First of all, an exchange of calcium for barium 
ions occurs on the surfaces of the calcium carbonate grains in the presence of barium 

9 



hydroxide. 



CaCOj + Ba(OH)2 = BaCOj + Ca(OH)2 
Exchange Pnx;ess 



During the second phase, relatively large crystals of barium carbonate are formed 

directly on those surfaces and molecularly bonded to them. Thus, the expected long-term 

consolidation will be produced. Carbonation of the stone surface results from the reaction 

of the barium hydroxide with atmospheric carbon dioxide, whereas carbonation of the 

interior of the stone takes place by the reaction of barium hydroxide with carbon dioxide 

which is released during the hydrolysis of urea. 

Ba(0H)2 + COj (atmospheric) = BaCOj + HjO 
Carbonation of Surface 

Ba(OH)2 + CO(NH2)2 (urea) = BaCOj + 2NH3 
Carbonation of Interior 

Crystal Growth Processes 

Lewin has suggested that the barium hydroxide technique is very suitable for certain 

types of calcareous stone^* and contends that it is safe and effective in consolidation when 

applied to an appropriate stone using a suitable technique.^^ Although the new method has 

been applied to a number of large projects in the field, it is still experimental. In the 

building industry, the markedly caustic nature of barium hydroxide is its major drawback 

since this may result in attack of other building materials including aluminum, zinc and 

glass.'*^ Discoloration may occur through the formation of white barium salts. 

1.3. ALKOXYSILANES: 

Alkoxysilanes have received the most attention recently due to their successful 
performance as stone consolidants. The use of alkoxysilanes for stone consolidation is not 
a recent development The possibility of using tetraethoxysilane (TEOS) or ethyl silicate for 
stone conservation was first suggested in 1861 by A.W. Hoffman.''^ It was produced as 

10 



an industrial chemical around 1924 and A. P. Laurie received a patent for its use as a stone 
preservative in 1925/^ During the 1960's, in Germany, extensive laboratory research and 
field tests with ethyl silicate were carried out and favorable results were obtained.''^ Since 
then, alkoxysilanes have been commonly used on deteriorated sandstones in Germany. 
Ethyl silicate and its combination with organo- silicon hydrophobic agents has been 
commercially available in western Europe since 1972 and thereafter in the United States and 
Canada.^'' These developments have brought a considerable interest to the use of 
tetraethoxysilane and also related molecular species, such as alkylalkoxysilanes and 
alkoxysUane-acrylic polymer mixtures, in stone consolidation. 

Alkoxysilanes are a family of monomeric molecules that have the ability to 
hydrolize with water to produce either silica or chains of alkylpolysiloxanes.^* The types 
of alkoxysilanes commonly used as stone consolidants are tetraethoxysilane (ethyl 
silicate or silicic acid ester), triethoxymethylsilane, trimethoxymethylsilane. When the 
alkoxysilanes are deposited in the stone, polymerization proceeds by two steps which are 
hydrolysis and condensation. At the end of that process siloxane linkages (- Si - O - Si -) 
are formed which provide the strengthening effect.'*' 

However, the formation of the final product is not very simple. There are several 
factors that control the rate of polycondensation and the structure of the forming products, 
such as the amount of water used in the reaction, the type of the catalysts, and the type of 
solvents.5° Alkoxysilanes with methyl groups (CH3) provide water repellency as well as 
consolidation. 

Even though alkoxysilanes have been commonly applied to the consolidation of 
sandstones, there have been attempts at using them on marble and limestone, as well.^' In 
addition, since 1969 ethyl silicate has been used in the consolidation of exposed adobe 
surfaces and predominandy satisfactory results have been observed to date.^^ 

Besides forming a binder similar to that in siliceous sandstone, alkoxysilanes have 

11 



the ability to penetrate deeply into porous stone because of their low-molecular weight.^^ 
On the other hand, their high cost and tendency to darken the color of the stone^** and the 
possibility of their evaporation from the surface before hydrolysis can take place^^ are some 
of the problems encountered in their use. Alkoxysilane treatments are irreversible. 
Alkoxysilanes will be discussed in detail in later chapters. 

1.4. SYNTHETIC ORGANIC POLYMERS: 

Since the use of synthetic organic polymers as stone consolidants is a recent 
development, dating back to the early 1960's, there is not much known of their long-term 
effects.^^ According to J. Riederer, consolidation with polymer materials works well for a 
considerable period, but not more than twenty years.^ 

Synthetic polymers are formed by polymerization of monomers which are the low- 
molecular weight compounds. There are two types of application methods of synthetic 
organic polymers to consolidate stone.'^ The simple procedure is to polymerize the 
monomeric organic molecules first, dissolve this polymer resin in an organic solvent and 
then apply it to the stone. When the solvent evaporates, the polymer remains in the stone. 
However, many solvents have a tendency to draw the dissolved resin back to the surface of 
the stone. In another application method, monomers either pure or dissolved in a solvent 
are polymerized in the pores of the stone after the solution has penetrated into the stone. 
Initiators and activators are also dissolved in the solvent to ensure that polmerization takes 
place after penetration. Deeper penetration can be achieved with monomer solutions, since 
they are less viscous than the diluted polymer resins and the size of a monomer molecule is 
very small.^' 

Although some synthetic organic consolidants significantly improve the mechanical 
properties of disintegrated stone, their weakness lies in the fact that they slowly deteriorate 
in the presence of oxygen and light.^ Discoloration, loss of tensile strength, and 

12 



brittleness are the symptoms of that process. Poor penetration is also among the 
disadvantages of synthetic organic polymers. Additionally, most synthetic resins have high 
thermal expansion coefficients larger than that of all stones and adobe.^' This negative 
factor can cause the development of a stress at the interface leading to possible detachment 
of the consolidated stone from untreated stone. Different types of decay regarding some 
stone structures in Germany, which had been consolidated with organic polymers, was 
reported by Riederer.^^ Deep channel erosion caused by rainwater occurred on the 
consolidated stone within ten years of the treatment. 

Synthetic resins can be divided into two main groups which are thermoplastic and 
thermosetting resins. They both have been used for stone consolidation, even though their 
structure is fundamentally different. Thermoplastics are composed of monomeric units, 
which are linked together by weak molecular forces to form two-dimentional linear 
chains.^3 They are reversible at least in principle and they remain soluble in appropriate 
solvents. However, they do not penetrate easily into small pores due to the large size of 
their molecules and tend to accumulate near the surface. The most widely used 
thermoplastic synthetic organic polymers are acrylates, polyethylene, nylon, and polyvinyl 
acetate. 

In contrast to thermoplastics, thermosetting resins are irreversible, because the 
monomeric units are linked together by strong chemical bonds to form a three-dimentional 
network.^ Once they set, they can not be remelted or reformed. Thermosetting resins are 
harder and stronger than thermoplastics, however they are also more brittie. Examples of 
thermosetting synthetic organic polymers are epoxies, polyesters, and polyurethanes. 

1.4.1. ACRYLIC POLYMERS: 

Methyl methacrylate and butyl methacrylate are the acrylic monomers that have been 
most widely used to consolidate stone and concrete. Several methods of polymerizing 

13 



acrylic monomers in situ have been investigated. They polymerize by means of heating 
with an initiator, ultraviolet radiation or gamma radiation.^' However, occasional 
destruction of the stone, such as cracking, is the serious problem which has been 
encountered during polymerization when the polymerization temperature is lower than 50°C 
or when using radiation. It seems that the formation of cracks is related to polymerization 
temperature. However, it is not clear that the type of monomer or initiator and presence or 
absence of solvents have an influence on the appearance of cracks. A study on the 
polmerization of methyl methacrylate in sandstone by P. Kotlik et al. concurs with the 
above findings.^ 

Depending on the type of initial monomer, the resulting polymer can have different 
properties. Methyl methacrylate monomers form quite hard polymers, polymethyl 
methacrylate (PMMA), while butyl methacrylate produces a relatively elastic product Stone 
consolidated with methyl methacrylate and other acrylic based polymers may exhibit a 
brittie behavior. If adequate impregnation and complete polymerization are achieved, 
methyl methacrylate can improve the mechanical properties of porous stone.^^ Acrylic 
resins have better resistance to oxidation and UV radiation than thermosetting resins. 

Acrylic resins have been used to consolidate sandstone, limestone and marble. 
Pre-polymerized methyl methacrylate in methylene chloride was tested to impregnate 
Egyptian limestone bas-reliefs, the Abydos reliefs.^ The reason for choosing this solution 
was that appropriate impregnation had been achieved with it on limestones. In this case 
however, penetration was not considerable and also a glossy coat formed on the surface. 

1.4.2. ACRYLIC COPOLYMERS: 

The joining of two or more different acrylic monomers, such as methyl acrylate, 
ethyl acrylate, methyl methacrylate, and butyl methacrylate, in a polymer chain forms the 
acrylic copolymers.^ The combination of different monomers rather than one monomer 

14 



variety intrcxluces the possibility of modifying the properties of the resultant polmer to fit 
the condition of the stone. For example, the addition of ethyl acrylate or butyl methacrylate 
to methyl methacrylate improves chemical strength, hardness and weathering.''" 

All the acrylic copolymers are dissolved in organic solvents and then applied to 
stone. The amount of penetration and actual impregnation resulting after the solvent 
evaporates depend on the choice and amount of solvent used. When too much solvent is 
used, evaporation will draw the acrylic copolymer back to the stone's surface forming a 
hard crust; when too little solvent or an improper solvent which evaporates too quickly is 
used, the acrylic copolymer will not penetrate deeply into the stone.'^' 

A copolymer of ethyl methacrylate and methyl acrylate, commercially known as 
Paraloid B-72 (Acryloid B-72 in US), has been widely used for surface consolidation of 
stone and reattachment of plaster and flaking stone in Italy .''^ The surface of the Carrara 
marble Lunette of the SS Giovanni and Paolo church in Venice was treated with a solution 
of Paraloid B-72 in nitro solvent as an interim measure to prevent further loss of 
disintegrated stone, in 1974.''3 G.Torraca indicates that Paraloid B-72 shows a good 
resistance to the ageing process and its life span is 3-10 years in external exposure.^'* De 
Witte et al. studied different polymerization methods on limestone by using Paraloid B-72 
in solution and a mixture of the monomers methyl methacrylate and ethyl acrylate, which 
are polymerized in situ under the influence of either gamma radiation or heating supposing 
that a polymer basically similar to B-72 would result.^^ It was concluded that the best 
results were obtained with the thermal polymerization of a mixture of monomers, whereas 
the treatment with Paraloid B-72 resulted in only 4% impregnation which cannot be 
considered successful. 

It is important to note that around 1975, the physical presentation of Paraloid B-72 
changed from white irregular lumps to regular transparent globulas. Although the 
manufacturer of Paraloid B-72 confirmed that there was no change in its chemical structure, 

15 



De Witte et al. claimed that the analysis of old and new Paraloid B-72 showed a small but 
definite difference between the two resins. They are both copolymers of ethyl methacrylate 
and methyl acrylate, but the old B-72 contains 2% more methyl acrylate than the new. 
Conservators should be aware of the fact that even this small difference in composition 
results in some changes in the physical properties of the resins.^^ 

Copolymers of acrylic/silane, acrylic/silicone, and acrylic/fluorocarbon have also 
been studied for stone conservation. Acrylic/silane mixtures will be discussed in Chapter 3. 
Gauri and Rao^'' reported that fluorocarbon-acrylic copolymers increase the resistance of 
stone to atmospheric attack, as a result of their study on epoxies, fluorocarbon-acrylics, 
and silicones. They recommended the use of fluorocarbon-acrylic mixtures for surface 
application rather than impregnation. Fluorocarbon-acrylic copolymers, however, are 
generally avoided in conservation practice today. 

1.4.3. EPOXIES: 

Ef)oxies have been used to consolidate limestone, sandstone, and marble in addition 
to their use as adhesives, paints, varnishes, and constituents of synthetic mortars and 
concrete. An epoxy resin system consists of an epoxide group and a curing agent or 
hardener, which modifies the physical properties and chemical structure of the resin and 
transforms it into an insoluble and infusible solid thermosetting cross-linked polymer. 
During hardening no by-products are released. Frequentiy used curing agents are primary, 
secondary and tertiary amines, amides, Lewis acids, phenols, anhydrides, Lewis bases, 
and inorganic hydroxides.^* Hardening can be achieved at room temperature or may require 
higher temperatures depending on the type of hardener. For example, aliphatic amines and 
amides are for room temperature use, whereas aromatic amines and acid anhyrides are for 
high temperature curing.^' The type and quantity of curing agent is rather important to 
obtain a well-hardened final product. A resin and hardening agent with low-molecular 

16 



weight is recommended to get sufficiently deep penetration into porous stone.*° 

Epoxies are generally too viscous to achieve significant penetration.Thereforc, they 
are usually dissolved in organic solvents to reduce the viscosity. The choice of solvent to 
be used with epoxides is also very important. The presence of solvents has an essential 
influence on the speed of the cross-linking reaction which should take place slowly to gain 
time for the diffusion of the resin molecules into the pores of stone. A solvent with 
hydroxyl groups (-OH) facilitates the cross-linking, whereas a solvent with a carbonyl 
group (IZ:C=0) in the molecule slows the hardening reaction down.*' The solvents also 
affect the color of the resulting produa. 

After epoxy resin impregnation, the resin tends to migrate back to the surface as the 
solvent evaporates. In the case of small objects, the epoxy impregnated stone can be 
wrapped with polyethylene or aluminum sheets to prevent premature evaporation of the 
solvent and reverse migration before cross-linking of the resin. This is not practical on 
large building siufaces. 

The most commonly used epoxy resins are epichlorohydrin and bisphenol A. 
Epoxy resins based on bisphenol A are found to protect the stone from carbon dioxide and 
sulfur dioxide, but are too viscous to penetrate stone deeply. *2 Their viscosity can be 
lowered by addition of low viscosity hardeners. Epoxy resins are now commercially 
available with satisfactorily low viscosities. 

Epoxy resins improve the mechanical properties of stone after correct hardening. 
They have a good adhesion to stone and are resistant to water and several chemicals. Two 
main drawbacks of the epoxies are their tendency to age and discolor (yellowing) upon 
exposiuie to light and their insolubility in ordinary solvents after setting. When exposed to 
sunlight, many epoxies tend to chalk, forming a white powdery surface.*^ Epoxy also 
tends to fill completely the pores of stone which is extremely undesirable, since apart from 
preventing air and water vapoiu* transmission through the treated stone, the stresses caused 

17 



by differential thermal expansion of epoxy will eventually deteriorate the stone.*'' Because 
of these negative features, the use of epoxies is not recommended for the surface 
consolidation of historic buildings. In conservation practice, epoxies are used most 
successfully as adhesives. 

1.5.WAXES: 

Waxes have been used to treat and protect stone for many centuries. In the first 
century B.C., impregnation of stone with wax was already described by Vitruvius. The 
most common natural waxes used for consolidating stone are beeswax and paraffin wax. 
Beeswax is obtained from the hives of honey bees.*^ Paraffins are petroleum-derived 
mineral waxes. They have been found effective in increasing the water repellency and the 
tensile strength of porous stone.*^ Microcrystalline wax, which is the modem equivalent of 
paraffin wax, has the same chemical composition but smaller crystals than those of paraffin 
waxes. Therefore, in contrast to brittie paraffin waxes, microcrystalline waxes have good 
plasticity and adhesion.*'' They appear to be effective as superficial consolidants. 

Waxes have been used in the form of a solution or in the form of melted mixtures. 
By either application, the stone is heated to keep wax molten during impregnation and to 
increase the depth of wax penetration into the stone. However, the high temperature needed 
to assist the penetration may damage the stone by causing, for example, spalling. Small 
objects such as statues and architectural museum pieces are impregnated by itnmersion in a 
bath of molten wax and deep penetration can be achieved. But, where total impregnation is 
not possible, depth of wax penetration into the stone is always very shallow. Therefore, 
waxes can be considered as protective coatings rather than impregnants. The other 
drawbacks of waxes are their tendency to soften at high temperatures and to collect dirt. 
Waxes may cause yellowing of the treated surfaces. In addition, the impregnated wax can 
rarely be removed when there is a need for application of other materials. 

18 



A wax-based protection has been recommended on non-porous stone and on good 
quality limestone.** Cosmolloid 80H, a commercial name for a microcrystalline wax, was 
successfully used as a surface coating on the non-porous Istrian limestone of the Loggetta 
of the Campanile, Venice.*' A paraffin wax with a low melting point was applied to 
Cleopatra's Needle in London to waterproof the surface of the granite. Unfortunately, it 
discolored the stone. Another failure has been reported in the use of a wax dissolved in 
turpentine on Westminster Abbey in London.'^ 

In general, waxes are no longer widely used or recommended for stone 
consolidation. 



19 



END NOTES : CHAPTER 1 



1. A. E. Grimmer, compiler, A Glossary of Historic Masonry Deteriorarion Problems and 
Preservation Treatments . Department of the Interior, National Park Service, Preservation 
Assistance Division (Washington, D.C. : U.S. Government Printing Office, 1984 ), p. 25. 

2. J. R. Clifton, Stone Consolidating Materials - A Status Report . NBS Technical Note 
1118, National Bureau of Standards ( Washington, D.C. : U.S. Government Printing 
Office, 1980 ), p. 1. 

3. G.G. Amoroso and V.Fassina, Stone Decav and Conservation: Atmospheric Pollution. 
Cleaning. Consolidation and Protection . Materials Science Monographs, 1 1 ( New York: 
Elsevier Science Publishers B.V., 1983 ), pp. 224-225. 

4. S. Z. Lewin, " The Preservation of Natural Stone, 1835-1965. An Annotated 
Bibliography, " Art and Archaeology Technical Abstracts 6, 1 (1966), pp. 185-277. 

5. H. Weber, " Conservation and Restoration of Natural Stone in Europe, " APT Bulletin 
XVn, 2 (1985), p. 15. 

6. M. B. Caroe, " Wells Cathedral Conservation of Figure Sculptures, 1975-1984, " APT 
Bulletin XVn, 2 (1985), pp. 3-13. 

7. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part I. Inorganic and Metal-Organic Compounds," in L. Lazzarini and R. 
Pieper, eds.. The Deterioration and Conservation of Stone . Notes from the International 
Venetian Courses on Stone Restoration, 1988, p. 291-294. 

8. W. Domaslowski and J. W. Lukaszewicz, " Possibilities of Silica Application in 
Consolidation of Stone Monuments, " in Deterioration and Conservation of Stone . 
Proceedings of Vlth International Ctongress, Torun, Poland, Sept. 12-14, 1988 ( Torun: 
Nicholas Copernicus University Press Department, 1988 ), p. 563; J. R. Clifton, Stone 
Consolidati ng Materials - A Status Report , p. 15. 

9. J. R. Clifton. Stone Consolidating Ma terials- A Status Report . p.21. 

10. G. G. Amoroso and V. Fassina, Stone Decav and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation a nd Protection , p. 308. 

11. W. Domaslowski and J. W. Lukaszewicz, " Possibilities of Silica Application in 
Consolidation of Stone Monuments," pp. 564-565. 

12. G. Torraca, Porous Bu ilding Materials: Materials Science for Architectural 
Conservation . 3rd edition ( Rome: ICCROM, 1988 ), p. 134. 

13. G. G. Amoroso and V. Fassina, Stone Decav and Conservation: Atmospheric 

20 



Pollurion. C leaning. Consolidarion and Protecrion . p. 308. 

14. G. Torraca, " General Philosophy of Stone Conservation, " in L. Lazzarini and R. 
Pieper, eds., The Deterioration and Conservation of Stone . Notes from the International 
Venetian Courses on Stone Restoration, undated, p. 251. 

15. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 309. 

16. C. S. Ewart, " Review of Stone Consolidants, " ICCROM, Rome, unpublished, 
1987, p.8. 

17. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " Art and Archaeology Technical Abstracts 18, 1 (1981), p.l53; 
H. Weber and K. Zinsmeister, Conservation of Natural Stone: Guidelines to 
Consolidation. Restoration and Preservation ( Ehningen bei Boblingen: expert - verlag, 
1991), p. 56. 

18. W. Domaslowski and J. W. Lukaszewicz, " Possibilities of Silica Application in 
Consolidation of Stone Monuments," pp. 563-576. 

19. Ibid., p. 569. 

20. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 310. 

21. J. R. Clifton. Stone Consolidating Materials- A Status Report , p. 18. 

22. G. Torraca, " General Philosophy of Stone Conservation," p. 251. 

23. J. R. Clifton, Stone Consolidating Materials- A Status Report, p. 18. 

24. G. A. Sleater, A Review of Natural Stone Preservation. NBSIR 74-444, National 
Bureau of Standards (Washington, D. C. : 1973 ), p. 12. 

25. J. Ashurst, " The Cleaning and Treatment of Limestone by the ' Lime Method ', Part 
1," Monumentum 27, 3 (1984 ), pp. 233-252. 

26. C.A. I*rice, " The Consolidation of Limestone Using a Lime Poultice and Limewater," 
in Adhesives and Consolidants . Preprints of the Contributions to the Paris Congress, Sept. 
2-8, 1984 (London: IIC, 1984 ), p. 160. 

27. J. Ashurst, " The Cleaning and Treatment of Limestone by the ' Lime Method ', Part 
1," p. 241. 

28. C. A. Price, " The Consolidation of Limestone Using a Lime Poultice and 
Limewater," pp. 160-161. 

29. C. A. Price, K. Ross, and G. White, " A Further Appraisal of the ' Lime Technique ' 
for Limestone Consolidation, Using a Radioactive Tracer," Studies in Conservation 33,4 
(1988), pp. 178-186. 



21 



30. S. Z. Lewin, The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds," p. 291; L Schnabel " 
Laboratory Assessment of the Barium Hydroxide-Urea Process for the Consolidation of 
Limestone and Marble," Thesis for Master of Science Degree, Graduate School of 
Architecture, Planning and Preservation, Columbia University, 1988, p. 23. 

31. S. Z. Lewin ^d N. S. Baer, " Rationale of the Barium Hydroxide-Urea Treatment of 
Decayed Stone, Studigf? in Con^fryarion 19, 1 (1974 ), p. 24; L. Schnabel, " Laboratory 
Assessment of the Banum Hydroxide-Urea Process for the Consolidation of Limestone 
and Marble, p. 24. 

32 G. G. Amoroso and V Fassina, Stone Decay and ConservaH pn: Atmn^ph^rir 
Pollution. Cleaning. Cons olidation and Protecrion . p ^11 ' 

33. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds, " p. 291- L Schnabel " 
Laboratory Assessment of the Barium Hydroxide-Urea Process for the Consolidation of 
Limestone and Marble," pp. 24-25. 

34. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds," p. 291. 

35. J. R. Clifton, Stone Consolidating Maf erials - A Status Report , pp. 19-21. 

36. S. Z. Lewin and N. S. Baer, " Rationale of the Barium Hydroxide-Urea Treatment of 
Decayed Stone, p.24. 

37. Ibid. 

38. L Schnabel " Laboratory Assessment of the Barium Hydroxide-Urea Process for the 
Consolidation of Limestone and Marble," p. 31. 

P?ii^' G-^™opso and y Fassina, Stone Decav and Conservatio n: Atmn^pherir 
Pollution. Cleaning. Con5>olid ation and Pir^terrinn p ^19 *^ "^ '^ 

40. S. Z. Lewin, " The Current State of the An in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds, " pp. 291-292. 

41 . S. Z. Lewin and N S. Baer, " Rationale of the Barium Hydroxide-Urea Treatment of 
Decayed Stone, pp. 24-25. 

42. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds, " p. 291. 

^^nln^- M'^Donald, " Coatings, Sealers, Consolidants, " Architecture . March, 1987 
pp. 78-79. ' 

44. C. A. Grissom and N R. Weiss, eds.. " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, p. 155. 

45. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation, " 
'" Vth International Congress on Deteriora ti on and Con.;ervation of Ston^ Volume 2, 

22 



Lausanne, Sept. 25-27, 1985, p. 831. 

46. K.J. H. Zinsmeister, N. R. Weiss, and F. R. Gale, " Laboratory Evaluation of 
Consolidation Treatment of Massillon (Ohio) Sandstone, " APT Bulletin XX, 3 (1988), 
pp. 35-36. 

47. H. Weber, " Conservation and Restoration of Natural stone in Europe," p. 15. 

48. J. R. Clifton, Stone Consolidating Materials - A Status Report , p. 22. 

49. C. S. Ewart, " Review of sTone Consolidants," pp. 27-34. 

50. W. Domaslowski and J. W. Lukaszewicz, " Possibilities of Silica Application in 
Consolidation of Stone Monuments," p.570. 

51. A. Moncrieff, " The Treatment of Deteriorating Stone with Silicone Resins: Interim 
Report, " Studies in Conservation 21, 4 (1976),pp. 179-191. 

52. G. Chiari, " Consolidation of Adobe With Ethyl Silicate: Control of Long Term 
Effects Using SEM, " in Alejandro Alva and Hugo Houben, coordinators, 5th International 
Meeting of Experts on the Conservation of Earthen Architecture . Rome, Oct. 22-23, 1987 
(Rome: ICCROM / CRAterre, 1988 ), pp. 25-29. 

53. M. Roth, " Comparison of Silicone Resins, Siliconates, Silanes and Siloxanes as 
Water Repellent Treatments for Masonry, " Technical Bulletin 983-1 ( Kansas City, 
Kansas: ProSoCo, Inc. ), p. 6. 

54. J. R. Clifton. Stone Consolidating Ma terials - A Status Repon. p. 24. 

55. C. V. Horie, Materials for Conservation: Organic Consolidants. Adhesives and 
Coatings ( London: Butterworth & Co. Publishers Ltd., 1987 ), p. 159. 

56. J. R. Clifton, Stone Consolidating Materials - A Status Report , p. 25. 

57. J. Riederer, " Recent Advances in Stone Conservation in Germany, " in E. M. 
Winkler, ed.. Decay and Preservation of Stone . Engineering Geology Case Histories, 11 
(Boulder, CO: The Geological Society of America, 1978 ), p. 91. 

58. J. R. Clifton, Stone Consolidating Mat erials - A Status Report, p. 24. 

59. G. A. Sleater, A Review of Namral Stone Preservation . NBSIR 74-444, p. 15. 

60. G. Torraca, Porous Building Materials , p. 90. 

61. G. Chiari, " Chemical Surface Treatments and Capping Techniques of Earthen 
Structures: A Long-Term Evaluation," in 6th International Conference on the Conservation 
of Earthen Architecture . Adobe 90 Preprints, Las Cruces, New Mexico, USA, Oct. 14-19, 
1990 ( Los Angeles: Getty Conservation Institute, 1990 ), p. 268; G. Torraca, " General 
Philosophy of Stone Conservation," p. 256. 

62. J. Riederer, " Recent Advances in Stone Conservation in Germany," p. 91. 

23 



63. G. Torraca, " Synthetic Materials Used in the Conservation of Cultural Property, " 
ICCROM, Rome, updated ( originally published in 1963 ), p. 306. 

64. Ibid. 

65. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation a nd Protection , p. 325. 

66. P. Kotlik, J. Ignas and J. Zelinger, " Some Ways of Polymerizing Methyl 
Methacrylate in Sandstone," Studies in Conservation 25,1 (1980), pp. 1-13. 

67. J. R. Clifton. Stone Consolidating Materials - A Status Report, pp. 26-27. 

68. A. E. Charola, G. E. Wheeler, and R. J. Koestler, " Treatment of the Abydos Reliefs: 
Preliminary Investigations, " in K. L. Gaiui and J. A. Gwinn, eds.. Fourth International 
Congress on the Deterioration and Preservation of Stone Objects . Louisville, Kentucky, 
July 7-9, 1982 (Louisville: The University of Louisville, 1982 ), p. 83. 

69. J. R. Clifton, Stone Consolidating Materials - A Status Report , p. 27 

70. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 328. 

71. C. S. Ewart, " Review of Stone Consolidants," p. 14. 

72. Ibid., p. 13. 

73. G. G. Amoroso and V. Fassina, Stone Dec av and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 337. 

74. G. Torraca, " General Philosophy of Stone Conservation, " p. 263. 

75. E. D. Witte, P. Huget, and P. Van Den Broeck, " A Comparative Study of Three 
Consolidation Methods on Limestone, " Studies in Conservation . 22, 4 (1977), pp. 190- 
196. 

76. E. D. Witte, M. Goessens-Landrie, E. J. Goethals and R. Simonds, " The Structure 
of Old and New Paraloid B-72," ICOM Committee for Co nservation 5th Triennial Meeting . 
Zagreb, 1978, pp. 1-9. 

77. K. L. Gauri and M. V. Appa Rao," Certain Epoxies, Fluorocarbon- Acrylics, and 
Silicones as Stone Preservatives, " in E.M. Winkler, ed.. Decay and Preservation of Stone . 
Engineering Geology Case Histories, 1 1, pp. 73-79. 

78. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection , p. 377. 

79. C. V. Horie, Materials for Conservation, p. 170. 

80. P. Kotiik, P. Justa, and J. Zelinger, " The Application of Epoxy Resins for the 
Consolidation of Porous Stone, " Studies in Conservation 28, 2 (1983), p. 78. 

24 



81. Ibid., p. 77. 

82. J. R. Clifton, Stone Cons olidating Materials - A Status Report , p. 28 ; P.Kotlik, P. 
Justa, and J. Zelinger, " The Application of Epoxy Resins for the Consolidation of Porous 
stone," p. 75. 

83. J. R. Clifton, Stone Consolidating Materials - A Status Report , p. 30. 

84. T. B. McDonald, " Coatings, Sealers, Consolidants," p. 78 

85. C. V. Horie, Materials for Conservation , p. 150. 

86. J. R. Clifton, Stone Consolidating Materials - A Sta tus Report , p. 31. 

87. G. G. Amoroso and V. Fassina, Stone Decay and Con servation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 314. 

88. Ibid. 

89. A. Moncrieff and K. F. B. Hempel, " Conservation of Sculptural Stonework: Virgin 
& Child on S. Maria Dei Miracoli and The Loggetta of the Campanile, Venice," Studies in 
Conservation 22, 1 (1977), pp. 8-9. 

90. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation a nd Protection , p. 318 



25 



CHAPTER 2 
ALKOXYSILANES 

2.1. PERFORMANCE CRITERIA : 

2.1.1. PERFORMANCE CRFTERIA FOR STONE CONSOLIDANTS : 

The effectiveness of a consolidation treatment depends on the proper selection of 
the consolidating substance, what is expected of it and how it is used. There are basic 
performance requirements that all consolidants must fulfill regardless of the specific 
application. The establishment of performance criteria for stone consolidants through years 
of experience provides a framework for evaluating any proposed consolidant and makes the 
selection of the appropriate consolidant easier for the conservator. 

The following criteria have been suggested by several authors for an effective 
consolidation treatment : 

o Consolidating Value : The primary requirement of a stone consolidant is to 
restore the physical integrity and mechanical properties of the decayed stone to an 
acceptable level by re-establishing the bonds between adjacent grains. A consolidant may 
perform this function through the deposition of a new and durable binding agent within the 
pores of stone. ^ 

o Durability of Consolidated Stone : The consolidated stone is expected to be as 
durable as the unweathered stone. A consolidant should form a compact front against 
atmospheric pollutants, dust, biological agents, and wind at least for some time.^ If the 
consolidated stone is less durable than unweathered stone, the replacement of the 
deteriorated stone with a new one is suggested.^ Furthermore, in terms of appearance, the 
weathering rate of the treated stone should be nearly the same as the untreated stone. 

26 



o Depth of Penetration : The consolidating substance should have the ability to 
penetrate easily and deeply into the weathered stone. Several authors have suggested that a 
stone consolidant should penetrate to such a depth that all the deteriorated zone is solidified 
and attached to the unweathered sound core of the stone. C. Price, on the other hand, has 
proposed the specific depth of penetration to be at least 25 mm.'' Ideally, a consolidant 
should penetrate uniformly into the stone pores and not accumulate at the surface forming a 
surface crust.^ 

4f There are many factors affecting penetration of the consolidant. They are related to 
the properties of the treating agent including its surface tension, viscosity, rate of 
evaporation and gel rate, as well as the characteristics and temperature of the porous 
material to which it is applied.^ Porosity, pore size distribution and moisture content are 
the most important properties of the stone that also control the penetration of the 
consolidant Low viscosity and high surface tension of the impregnating fluid are desirable 
to achieve good penetration. 

o Stone Porosity : The consolidant should not adversely affect the pore structure 
of the stone. Treating stone with consolidating material can reduce the mean size of the 
pores and make the stone more susceptible to frost and salt damage due to the increased 
proportion of fine pores.' 

o Moisture Transfer : Consolidants should be water vapor transmissive so that the 
treated stone can allow water vapor and air circulation to prevent the accumulation of 
moisture and salts behind the treated layer which will ultimately cause the deterioration of 
the stone. Therefore, complete filling of the pores with an organic hydrophobic consolidant 
is not recommended.^ 

o Compatibility of Consolidant with Stone : The consolidant should have similar 
thermal-dimensional properties as the untreated stone. It should not have deleterious 
chemical or physical interactions with the stone which can form harmful by-products such 

27 



as soluble salts. Similarly, it should not disrupt the microstructure of the stone, for 
instance, through crystal growth from the precipitation of an inorganic consolidanL When 
the thermal expansion of consolidated stone is very different from untreated stone, it may 
produce important tensile stresses leading to cracking at the interface between the 
consolidated and untreated stone.' The crystal growth of precipitate may also produce 
internal tensile stresses causing microcracks and eventual macrocracks in the stone. 

o Effects of Consolidant on Appearance : The consolidant should create no or 
minimal alteration in texture, color and reflectance of the stone either initially or after 
exposure to the environment If the treatment alters the natiual appearance of the stone, the 
conservator responsible for the preservation of the structure will have to evaluate its 
acceptability. 

o Ease of Application : During handling or application, a consolidant should not 
introduce health and safety hazards. If the consolidant is toxic, volatile, flammable, etc., 
adequate safety precautions should be taken for applicators which includes use of protective 
clothing, goggles and respiratory masks. All consolidants should be applied by experienced 
applicators. Environmental considerations regarding protection of surrounding landscape 
and sidewalk areas should also be taken into account. 

In addition to the primary performance requirements, a specific application may 
necessitate secondary requirements, as well. These requirements may be encapsulation of 
salts or extraction of salts from stone, prevention of further microbiological growth, 
providing water repellency, reversibility to allow future treatment, etc. 

It is important to note that the reversibility of a consolidation product is considered 
by many conservators to be more theoretical than practical. •" Even if the treatment is 
reversible, it is an impractical and difficult operation to remove a consolidant from 
immovable stonework on buildings, especially after it has been in place for a certain period 
of time. The consolidation treatment may not be reversible, but may be retreatable so that 

28 



the future application of the same treatment or another treatment may be possible as it is 
needed. The cost also should not be disregarded in selecting the appropriate consolidanL 

In 1921, the characteristics of an ideal stone preservative had already been 
described by Heaton." It can be easily seen that his criteria are still relevant and valid to 
stone consolidants today : 

1. It must penetrate easily and deeply into the stone, and remain there on drying. 

2. It must not concentrate on the surface so as to form a hard crust, but must, at 
the same time, harden the surface sufficiently to resist erosion. 

3. It must prevent penetration of moisture, and, at the same time, allow moisture to 
escape. 

4. It must not discolour or in any way alter the natural appearance of the stone. 

5. It must expand and contract uniformly with the stone so as not to cause flaking. 

6. It must be non-corrosive and harmless in use. 

7. It must be economical in material and labour of application. 

8. It should retain its preservative effect indefinitely. 

However, there is no one product that is able to fulfill all the requirements for stone 
consolidation at the same time, because these requirements can be contradictory. A variety 
of inorganic, organic and organosihcon consolidants are commercially available today, each 
accompanied by scientific data regarding production and use. Different cases will require 
different products and treatments and some no consolidation at all. 

2.1.2. HOW ALKOXYSILANES MEET THE PERFORMANCE CRITERIA: 

Alkoxysilanes have received increased attention as stone consolidants and are 
considered by many conservators to be the most promising stone consolidating materials 
available today. What makes the alkoxysilanes so attractive is that some of them, notably 
the alkylalkoxysilanes and silicic acid esters, meet several of the performance 
requirements. 

One of the most important properties of alkoxysilanes is their ability to penetrate 
deeply into porous stone. This advantageous property differentiates alkoxysilanes from 
their many predecessors. Their low viscosity and small constituent monomeric molecules 

29 



give the alkoxysilanes a substantial penetrating power. They can penetrate certain porous 
stones to a depth of between 20 to 25-30 mm,'^ thus eliminating the problems of shallow 
treatments. 

When alkoxysilane monomers are polymerized, the structure of the cured polymers 
provides chemical stability through strong silicon-oxygen-silicon bonds and produces a 
high strengthening effect. Alkoxysilanes can cure at normal outdoor temperatures. '^ 
Hydrolysis reactions of the alkoxysilanes produce alcohols as by-products which are not 
deleterious to stone and evaporate as soon as they are generated, thus leaving the solid 
polymer in the stone.''' It has been reported that solidified material does not seem to fill the 
entire pore space, but instead coats the pores, therefore permitting the transmission of 
moisture. This would also allow the treated stone to absorb a further treatment when 
necessary. 15 Alkoxysilane consolidation is not reversible, but permits retreatment'^ 

On the other hand, alkoxysilanes do have disadvantages. They have been reported 
to cause some slight changes in the color of treated stone, such as an initial darkening, i'' 
and also formation of white spots on the stone.'* Alkoxysilanes are likely to evaporate 
from the surface before hydrolysis can take place. '^Additionally, alkoxysilanes are 
expensive materials and the consumption of material is high in order to achieve deep 
penetration.20 High cost may, thus, restrict their use to small objects and limited surface 
areas. Necessary precautions in application are not convenient for large projects. All of the 
alkoxysilanes may be regarded as hazardous for human health. They can cause kidney 
damage and injury to the eyes leading to blindness, if proper safety precautions are not 
taken. 

2.2. CHEMISTRY OF ALKOXYSILANES : 

Alkoxysilanes are monomeric organo-silicon compounds containing silicon, 
oxygen, carbon and hydrogen atoms. Most of these compounds are colorless, low 

30 



viscosity liquids with relatively low toxicity^' and moderate to low volatility.22 They react 
with water to give a solid polymer varying in nature from silicone resin to fine powder or 
glassy silica. The physical properties and chemical composition of the resulting product 
depend on the specific conditions which exist during the total process that will be ftirther 
discussed in this section. 

2.2.1. CHEMICAL STRUCTURE OF ALKOXYSILANES : 

Alkoxysilane refers to a silane containing at least one alkoxy group. A silane is a 
compound based on a silicon backbone to which the alkyl groups and/or alkoxy groups are 
attached directly through a silicon-carbon link (Si-C, silane link) or an ester link (Si-O-C). 
Some alkoxysilanes can contain both silane and ester links at the same time, such as 
triethoxymethylsilane and trimethoxymethylsilane. 

An alkyl group is a compound of carbon and hydrogen atoms, such as methyl (- 
CH3) and ethyl (-C2H5) groups. Those hydrocarbon groups are often represented by the 
letter R. An alkyl group bonded to an oxygen atom forms an alkoxy group, such as 
methoxy (-OCH3) and ethoxy (-OCj^s)- 

It should be emphasized that the term silane was originally applied to silicon 
hydrides that are the compounds formed by silicon and hydrogen. Since then, silane has 
been used to refer to many of the monomeric organo-silicon compounds including ethyl 
silicate.^ 

Alkoxysilane is used here as a principal term describing the major types of 
consolidants that are commonly used to consolidate stone. A list of commercially available 
products in each category is presented in Appendix A. 
1. ETHYL SILICATE (TETRAETHOXYSILANE) [ Si(OC2H5)4] : 

Ethyl silicate is the best known alkoxysilane, also referred to as tetraethoxysilane. 
Silicic acid ester, silicon ester, tetra(ethyl)orthosilicate (TEOS) have also been used in the 

31 



literature to refer to ethyl silicate. Ethyl silicate is produced industrially by the reaction of 
silicon tetrachloride (SiCl4) with ethyl alcohol (CjHjOH).^^ Its chemical structure may be 
represented as: 

0(C2H5) 

I 

(H5C2X) — Si — OCCjHj) 

I 

Ethyl silicate has an extremely low viscosity (0.60 centipoises at 20°C)25 that is 
lower than the viscosity of water (1.0 Nsm-^xlO"^ at 20°C).^ Thus, ethyl silicate is more 
mobile than water. This desirable property for a consolidant provides ethyl silicate with 
excellent penetration in porous materials. However, large quantities of the material are 
required in order to get sufficient penetration which makes the treatment rather expensive.^^ 
Ethyl silicate is volatile, so conditions must be controlled to prevent excessive evaporation 
before consolidation takes place.^* 

2. PARTIALLY POLYMERIZED ETHYL SILICATE : 

Ethyl silicate that is partially hydrolized and condensed to produce small polymers 
enable of further polymerization is called partially polymerized ethyl silicate. It is produced 
by including water during the synthesis of ethyl silicate. Partially polymerized ethyl silicate 
is widely marketed. It is called " ethyl silicate 40 " in the US which indicates the specific 
degree of polymerization. 

The liquid may be yellow or brown and more viscous than the monomeric ethyl 
silicate. Partially polymerized ethyl silicate is less expensive than monomeric ethyl 
silicate.29 

3. TRIETHOXYMETHYLSILANE [ (CH3)Si(OC2H5)3 ] : 

This compound is chemically similar to ethyl silicate except for the substitution of a 
methyl group for one ethoxy group. The term is sometimes used in inverted order as 
methyltriethoxysilane (MTEOS). 

32 



Its chemical structure may be represented as : 

0(C2H5) 

I 

H3C — Si — ©(QHs) 
I 
OCCzHs) 

The viscosity of triethoxymethylsilane (0.6 Nsm^xlO-^ at 20°C) is lower than that 
of water (1.0 Nsm^xlO-^ at 20^0. 3° In addition to their consolidating effect, 
triethoxymethylsilanes also provide water repellency through the exposed methyl groups of 
the cured product. 
4. TRIMETHOXYMETHYLSILANE [ (013)81(0013)3 ] : 

Trimethoxymethylsilane is structurally similar to triethoxymethylsilane except for 

the substitution of methoxy groups for the ethoxy groups. Some authors use the term in 

inverted order as methyltrimethoxysilane (MTMOS). Its chemical structure may be 

represented as : 

0(CH3) 
I 
H3C — Si — 0(CH3) 
I 
0(CH3) 

Trimethoxymethylsilanes are also low viscosity monomers ( viscosity 0.5 Nsm"^ 
x 10-3 at 20°C )3' and provide water repellency as well as consolidation. However, 
trimethoxymethylsilane may be toxic.^ 

Methyl(trialkoxy)silanes or, more generally, alkyl(trialkoxy)silanes, have been used 
to refer to triethoxymethylsilane and trimethoxymethylsilane as well. They may be more 
expensive than ethyl silicate. 

2.2.2. POLYMERIZATION OF ALKOXYSILANES : 
HYDROLYSIS AND CONDENSATION 

The application of alkoxysilanes to stone consolidation is based upon the fact that 

33 



the initial liquid compound can be converted into a consolidating solid deposit within the 
stone through interaction with either liquid water or water vapor. ^3 When the alkoxysilane 
is applied to stone as a monomeric molecule, its polymerization is initiated by a hydrolysis 
reaction. 

Hydrolysis is the chemical reaction of an alkoxysilane with water or with the 
hydroxyl groups (-0-H) on the surface of a mineral grain. Several types of stone, brick and 
clay have reactive hydroxyl groups on their surface. The alkoxy groups (-0-R) of 
alkoxysilanes are capable of reacting with hydroxyl groups. 

The hydrolysis reaction produces a silanol and an alcohol as a by-product which is 
hannless to stone and rapidly evaporates. In the course of polymerization, this partially 
hydrolized molecule can then undergo either further hydrolysis or condensation. 

Two silanol molecules, which are the products of the hydrolysis reactions, can react 
with each other and condense to form a dimer molecule. Water is produced as well. Further 
hydrolysis and condensation proceed simultaneously and beyond the dimer stage trimers, 
tetramers and eventually a network of polymers having a -Si-O-Si-O-Si- backbone, called 
siloxanes, is formed. It is the siUcon-oxygen-silicon linkages that produce the consolidation 
and strengthening effect.^ 

An alkoxysilane is a mobile liquid. As hydrolysis and condensation reactions 
proceed, the liquid changes into a glassy solid. The network polymer that first forms is a 
soft gel; the gel shrinks and becomes harder; the shrinkage continues and it changes into a 
hard, brittie glass; in the end, it shrinks to small, glassy particles or crumbles to a fine 
powder.35 

If the alkoxy groups react with the hydroxyl groups present on the surface of the 
stone grains, they become molecularly bonded to the substrate. One end of the siloxane 
chain is bonded to the surface of one mineral grain, while the other end is attached to an 
adjacent grain. If the reaction takes place with the hydroxyl groups of water molecules, the 

34 



resulting network of polymers is not bonded to the grains of the stone, but fill the 
intergranular spaces of the stone. There are conflicting opinions among conservators and 
scientists regarding the importance of the molecular bond in strengthening stone. According 
to A.E.Charola et al., the lack of chemical interaction with the substrate may be an 
advantage.^ 

When alkoxysilanes with alkyl groups [alkyl(trialkoxy)silanes] polymerize, unlike 
the Si-O-C link (ester link), the Si-C link (silane link) resists hydrolysis and the final 
product retains alkyl groups attached to the siloxane backbone. It is the alkyl groups that 
present a non-polar aspect to the surroundings and therefore provide water repellency in 
addition to the consolidating effect of the alkyl(trialkoxy)silanes. For instance, 
trimethoxymethylsilane is polymerized as follows: 
1. The methoxy groups are hydrolyzed to form a silanol, and methanol is liberated: 



OCH3 OCHj 

I . . I 



H3C — Si 

I 



OCH, + H 



OH — > H3C — Si — OH + CH3OH 
I 
OCH3 OCH3 

Trimethoxymethylsilane + Water Methyl Dimethoxy Silanol + Methanol 

2. Two methyl dimethoxy silanol molecules react and condensation takes place. As a result, 

silicon-oxygen-silicon bonds arc formed and water is released: 

OCH3 OCH3 OCH3 OCH3 

I . 1 I I I 



H,C — Si — 



OH + HO 



— Si — CH3 — > H3C — Si — O — Si — CH3 + H2O 
I I I 

OCH3 OCH3 OCH3 OC3i3 



It is ^parent that water is produced and consumed in the course of polymerization. 
3. Further hydrolysis and condensation reactions convert -Si-O-C linkages into -Si-O-Si- 
linkages and generate a three dimentional cross-linked polymer which has some similarity 
to the chemically very stable compound, silica: 

35 



CHj CH3 CH3 O 

I I I I 

O — Si — O — Si — O — Si — O — Si — CH3 
I I I I 

0000 
I I I I 

CH3 — Si — O — Si — O — Si — O — Si — O — 
I I I I 

O CH3 CH3 CH3 

I' 
It is easily seen that methyl groups are left exposed and provide water repellency. 

The conversion reaction of alkoxysilane monomers into a cross- linked polymer 
does not occur spontaneously; certain conditions are necessary for the course of the 
reaction. These are: 

o Presence of Water: One of the most important conditions is the presence of water, 
since the partial reaction of the alkoxysilane liquid with water initiates polymerization. It 
has been suggested that sufficient water must be present in the initial solution along with 
the silane to produce gelation, because one can not rely on the hygroscopic moisture 
content of the building material itself, or atmospheric humidity to achieve a complete 
reaction.37 The amount of water used in the reaction markedly affects the mechanism of the 
gel formation. As a result of their study on the polymerization of tetraethoxysilane, 
S.Z.Lewin and G.E.Wheeler have suggested that the molar ratio of water to silane should 
be at least 2: 1 to produce gelation. They also reported that even the order of mixing one in 
another gives a different set of reaction products. For example, tetraethoxysilane added to a 
water-alcohol solution gives a more predictable produa than water added to a silane-alcohol 
solution.38 

o Solvents: The solubility of water in alkoxysilanes or alkoxysilanes in water is 
very small, therefore it is necessary to employ a mutual solvent in the starting solution. For 
instance, ethyl alcohol is often used as the mutual solvent to achieve the miscibility of ethyl 



36 



silicate and water.^' Methanol and cellosolve are two examples of solvents which are used 
with methyl(trialkoxy)silanes (MTEOS,MTMOS)/water mixture.'**' Solvents arc frequently 
added to alkoxysilanes to reduce their viscosity as well.'" In this way, the depth of 
penetration into the stone can be controlled. 

o Catalysts: Catalysts are often employed to speed up the rate of hydrolysis and 
condensation reactions so that the evaporation of the monomer before polymerization is 
avoided. In the absence of catalysts, the polymerization is too slow for practical field use. 
Acids (e.g. especially hydrochloric acid and phosphoric acid), alkalis and metallic salts 
have been used as catalysts for both reactions. Solvent and catalyst choice is important. 
When choosing a solvent and catalyst to increase the rate of polymerization, one should be 
aware of the fact that they must not react with the substrate and leave any soluble residue in 
it which can cause future decay. 

The rate of polymerization has an essential impact on the structure and quality of the 
forming product. The penetration of the impregnating liquid into the stone's pore system 
requires a time period varying from several hours to a day. However, if the liquid is too 
volatile, it will evaporate before a sufficient penetration is achieved. On the other hand, if 
the polymerization is too fast, gelation will occur before an effective penetration is obtained 
and consolidation will occur only at the surface, creating a potential source of future failurc. 
Adjusting and controlling the rate of polymerization is obviously the most complicated and 
difficult problem in the application of alkoxysilanes to stone conservation. Besides 
employing catalysts and solvents, some other factors also influence the polymerization rate 
such as the type of the hydrocarbon group (methyl, ethyl,etc.) that is attached to the silane 
molecule^2 j^d the relative humidity at which the reaction is carried out.^^ The rate of the 
hydrolysis reaction is slower when the hydrocarbon group is larger and bulkier.^ 

An experiment on uncatalyzed polymerization of trimethoxymethyl silane, carried 
out by A.E.Charola et al. .revealed that the rate of polymerization and the quality of the 

37 



polymer are sensitive to the relative humidity (RH).''^ The relative humidity should be 
between 50% and 30% for the application of trimethoxymethyl silane. Above 50% RH, the 
rate of hydrolysis is too rapid, and during condensation, stresses are developed leading to 
cracks and defects. Below 30% RH, the reaction rate is too slow, permitting excessive 
evaporation of the silane. The evaporation of liquid monomer has also been found to be RH 
dependent. More trimethoxymethylsilane evaporates at lower RHs. However, it is 
extremely difficult to control relative humidity in the field. Even though the ambient RH 
may be ideal, the moisture content of the porous stone varies at different depths, thus 
intnxlucing a non-homogeneous f>olymerization environment 

For the effective application of alkoxysilanes in conservation, in addition to 
providing appropriate conditions for the reaction, application of the impregnating liquid on 
an appropriate type of stone is very important. The physical and chemical nature of the 
substrate also influence the polymerization process. Alkoxysilanes do not give the same 
results with all types of stone. As mentioned before, the presence of reactive -OH groups 
on the mineral grains of the stone can lead to a molecular attachment of the siloxane chains 
to the mineral grains. Thus, it has been claimed that the alkoxysilanes are most effective for 
porous, fine-grained, weak stones with surface hydroxyl groups, such as many types of 
sandstone and clay rich stones.^^ According to many conservators, mudbrick and other 
earthen substrates are particularly suited to consolidation with ethyl silicate.^' 

Nonetheless consolidation of calcareous stones, such as limestone and marble, with 
alkoxysilanes has been achieved. Although the chemical affinity of silanes for calcareous 
stones is minimal, a network of polymer may fill the intergranular spaces of the stone and 
having a consolidating effect without forming a chemical bridge between the grains. 

The mechanism of alkoxysilane polymerization is very complicated and is still not 
completely understood. 



38 



2.3. APPLICATION METHODS: 

An alkoxysilane consolidant is usually sprayed with low pressure or brushed on to 
the substrate in repeated applications referred to as " cycles " by some manufacturers. The 
number of applications and the waiting time between cycles or applications can vary for 
different products, therefore each product should be applied in accordance with the 
manufacturer's recommendations. In the initial stages, applications are absorbed very 
rapidly. Further applications should normally take place until no more is taken up. When 
necessary, absorbent pads of tissue should be placed below the treatment area to prevent 
excess silane from running down. Adjacent surfaces not designated for treatment and/or 
non-masonry surfaces should be protected from overspray of consolidant with sheets of 
polyethylene fixed firmly to these surfaces or by other protective materials such as adhesive 
tapes, non-oily modeling clay, etc. Following final application of the consolidant, 
excessive material should be washed immediately with a cleaning solvent ftx)m the masonry 
surface in order to prevent a possible surface discoloration. 

Choosing the right application method very much depends on the type of surface to 
be consolidated, the porosity of the material and its dimension. Application by hand 
operated spray is very convenient and least damaging to friable surfaces, whereas 
application by brush is suitable on very firm surfaces. In addition, small surfaces can be 
treated with simple spray bottles, while larger surfaces require spray devices, such as 
spray-guns, etc. 

Other application methods for alkoxysilanes include immersion of the substance in 
the consolidating solution, absorption by capillary suction and the use of compresses. 
Immersion can be accomplished under vacuum"** or at atmospheric pressure. Obviously, 
this method is only applicable to mobile and moderate size objects such as sculptures and 
architectural pieces. 

During the course of the immersion technique at atmospheric pressure, the object is 

39 



completely immersed in a basin filled with the consolidanLit is recommended that in order 
to get the maximum depth of penetration the object should be removed from the bath 
occasionally.'*' This will allow air which is trapped within the pore spaces to escape and 
provide a homogenous distribution of the consolidant. After immersion.the impregnated 
object is covered with an impermeable wrapping to avoid the evaporation of the 
impregnant. This wrapping must be kept in place from several days to a week depending 
on the case.5°The advantage of the immersion techique utilizing vacuum is that 
substantially deep penetration can be achieved. In order to increase the absorption of 
consolidant, a gas pressure (e.g. nitrogen gas) can be applied to the system. 

The use of compresses is also suitable for mobile objects. The object is wrapped in 
a poultice and sealed air tight by means of a polyethylene film. The consolidant is injected 
into the poultice at a slow rate. 

Certain specific conditions should be provided before the application takes place. 
Alkoxysilanes should be applied to a clean, dry and absorbent surface to get an adequate 
penetration. Obviously, a wet masonry substrate whose pores and capillaries are full of 
water will not have absorbency for the impregnated agent. Therefore, the consolidant 
should not be applied during rain or when there is chance of rain within 24 hours after 
application. In addition, rising damp problems should be corrected before consolidation. 
Control of other surface conditions for example, surface and air temperatures, wind and 
sun exposure is essential for proper performance of an alkoxysilane. Prior to application, 
masonry to be treated should be protected from direct sun radiation to keep the surface 
relatively cool and prevent rapid evaporation of the alkoxysilane. Application should be 
avoided when wind is sufficient to carry airborne chemicals to unprotected surfaces. For an 
adequate penetration, surface and air temperatures should not be too warm or too cool. It 
has been suggested that air and surface temperatures should be from 50 degrees F to 85 
degrees F during application.^' 

40 



Clean surfaces will improve the absorbency of the masonry. Surface contaminants 
such as salts, carbon crusts, atmospheric stains, and bird droppings must be removed to 
assure thorough penetration. Additionally, all repairs to the masonry should be completed 
before the consolidating agent is applied because treated surfaces will not bond to 
cementitious or synthetic repair materials. The repair work may include pinning, mortar 
filling, fracture grouting, repointing, adhering detached pieces, and application of biocides, 
etc. It is imp>ortant to know that silanes will not fill large cavities nor bridge large gaps. 
They can be used only for specific aspects of a conservation project 

Most importandy, alkoxysilane consolidants should only be applied by trained 
applicators who are familier with these types of chemical preparations. During the 
application of alkoxysilane consolidants, adequate precautions should be taken to avoid 
contact with skin and eyes. Safety glasses should be worn at all times for eye protection. 
In addition to protective clothing, the use of rubber gloves for the applicators is 
recommended because the silane has a degreasing effect on the skin. The alkoxysilane is 
volatile and should not be inhaled. TTierefoie, the use of a respiratory mask is necessary. 
(Obviously, the material must also not be swallowed or ingested, since it is toxic.) 
Smoking must be banned in the working area, because alkoxysilane is flammable. 
Surrounding landscape, lawn and sidewalk areas should be protected from contact with 
cleaiting materials and consolidating treatments through the use of polyethylene sheeting or 
these areas should be continuously washed with a steady mist of water. 

2.4. PROTECTION AND MAINTENANCE: 

If an alkoxysilane does not provide water repellency along with its consolidating 
effect, treated surfaces will be susceptible to weathering influences by absorbing water. 
Application of a water-repellent material might be indicated to protect treated surfaces from 
rain or damaging moisture. Alkoxysilane consolidants have a limited service life like other 

41 



conservation treatments. After a certain time period, re-application of the same consolidant 
or a different one will be required for a continuous protection of the masonry substrate. 

Above all, buildings, monuments or sculptures can not be preserved by a single 
treatment Regular inspection and maintenance of all protective systems are also necessary. 
Routine maintenance includes improvement of all the systems of protection against 
rainwater such as roof coverings, gutters, downpipes and flashings, control of rising damp 
in masonry, repointing of joints with a suitable material, cleaning, removal or control of 
vegetation, re-application of plaster if it had been used originally for protection of the 
masonry or earthen substrate. Moreover, careful records of the inspections including 
monitoring of the consolidated area and detailed description of the treatments which have 
been applied should be collected in a maintenance manual which establishes a maintenance 
program for future conservation references. 

2.5. EVALUATION TECHNIQUES: 

Prior to the application of alkoxysilanes or any other consolidation treatments, it is 
necessary to conduct a number of laboratory and field tests to understand the ongoing 
deterioration processes and to identify the most appropriate consolidation treatment for the 
masonry conditions present at the job site. Although the long-term effectiveness of the 
reported alkoxysilane applications will help to evaluate them as masonry consolidants, it 
can not be assumed that their overall performance will be similar for each project which 
shows a unique set of problems and requirements due to differences in environment, type 
of stone, degree of stone decay, etc. Additionally, because the value of a preservative is 
proven with time, there is a lack of performance data for the materials that have recentiy 
been used as consolidants. Therefore, there is a need for laboratory test programs to 
provide the rapid evaluation of any consolidation treatment. Following the complete 
laboratory tests, on-site test applications are also necessary to confirm the laboratory test 

42 



results and specify applicarion procedures. 

2.5.1. LABORATORY TESTING PROGRAM: 

Laboratory testing programs are designed to assess the physical and chemical 
properties of the stonework and to identify the factors contributing to its decay. 
Furthermore, they assist in evaluating the ability of materials to meet the performance 
criteria for stone consolidants so that the number of available consolidants can be reduced 
to one or two most promising alternatives. Besides providing a short-term evaluation of the 
recommended consolidation treatment(s), laboratory programs have the advantage that test 
conditions can be adjusted to simulate the specific environment and climate to which the 
consolidant will be exposed.^^ xhe laboratory test program is conducted in two phases: 

1. A complete laboratory evaluation of the untreated masonry samples. 

2. A complete laboratory evaluation of the treated masonry samples. 

The comparison of the test results derived from tests run on treated and untreated stone 
samples is the most practical method of evaluating the effectiveness of stone consolidants.^^ 
Such a comparison will demonsrate how well a consolidation treatment maintains the 
properties of the treated samples relative to the untreated samples and evaluate the 
improvement to the material offered by the recommended treatment. There are various test 
methods which have been developed to measure the durability and physical and chemical 
properties of natural stone and the effects of exposure to weathering factors. Some of these 
methods have been standardized by the American Society for Testing and Materials 
(ASTM), the Deutsche Industrie Norms (German Industrial Standards - DIN), and RILEM 
Committee (Reunions Internationales des Laboratoires d'Essais et de Recherches sur les 
Materiaux et les Constructions). A list of these standard test methods which might be used 
in a laboratory test program is presented in Appendix B. 

Selection of samples is critical to a proper laboratory evaluation. For a complete and 

43 



reliable laboratory evaluation, samples should represent the variety of decay processes. 
Therefore, samples should be taken from weathered stone at various locations where 
degree of stone decay and condition of the stone is different. The number of samples 
provided for laboratory evaluation is also important to get a comprehensive evaluation. The 
conservator responsible for the preservation of the structure should be able to judge 
whether the number of samples submitted for laboratory testing is representative enough of 
the condition of the masonry. The form and size of the samples for testing can vary 
depending on the test method Drill core samples are usually taken from the representative 
areas. Larger sized specimens, like test bars, are required by some test methods established 
by ASTM Standards. On the other hand, E.M. Winkler has introduced thin discs for testing 
the durability and strength of stone.** He suggests that the use of small quantities of stone 
for testing is preferable, because the natural heterogeneity of the stone can lead to 
inconsistencies in test results. In addition, problems can be faced regarding the total 
penetration of most stone consolidants when large sized test samples required by some 
ASTM standard test methods are used. 

The following tests are recommended for a comprehensive and reliable laboratory 
evaluation of untreated masonry samples to determine the suitability of the masonry for 
consolidation treatment:^^ 

o Petrographic Analysis: The material composition of untreated masonry is 
identified through optical microscopic evaluation and x-ray diffraction analysis which is 
necessary to determine the appropriateness of the masonry substrate for chemical 
consolidation treatment 

o Water Solubility: The percent water-soluble contents of untreated masonry 
samples is determined. Such information is utilized in identifying the vulnerability of the 
masonry to water-related deterioration. 

o Acid Solubility: The percent acid-soluble contents of untreated masonry samples 

44 



is determined to identify the masonry's vulnerability to acid-related deterioration when 
exf)osed to acidic precipitation. 

o Water Absorption: The water absorption capacity and rate are measured for each 
sample and may be correlated to the masonry's available pore capacity for consolidation 
treatment. This information is utilized in selection of the most suitable consolidation 
treatment(s) and application procedures. 

o Hygroscopic Moisture Uptake: The hygroscopic characteristics of untreated 
masonry samples is determined. Such information assists in identifying the kind of natural 
cements and foreign matter in the masonry samples. 

o Anionic Salt Analysis: The percent concentrations of chloride, sulfate and nitrate 
salts present in each masonry sample are determined. Knowing the nature and distribution 
of salts in the masonry samples helps to identify the nature and cause of ongoing 
deterioration. 

Once the above testing procedures have been completed, evaluation of the 
laboratory data in conjunction with the survey of existing conditions of the masonry can 
assist in identifying the causes of decay, the suitability of the substrate for consolidation 
and finally in selecting the appropriate consolidant(s) for the masonry conditions present at 
the job site. 

Masonry samples treated with the selected consolidant(s) are then analyzed to 
determine the performance of the treatment. Consolidating and penetrating abilities of the 
consolidants, as well as their effect on the appearance and physical properties of the 
masonry substrate are evaluated. Comparison of treated and untreated samples helps to 
evaluate the change in the characteristics of the masonry treated with the consolidant. The 
following tests are suggested for the laboratory evaluation of treated masonry samples:* 

o Water Solubility 

o Acid Solubility 

45 



o Water Absorption 

The above tests are repeated with treated samples in the same way as described for 
untreated samples. Treated and untreated samples are then compared to evaluate the change 
in the characteristics of the masonry caused by the consolidating treatments. 

Other tests recommended for the treated samples include: 

o Absorption of Treatment: The amount of consolidant deposited in each of the 
treated masonry samples is determined to evaluate the effectiveness of the treatment and 
efficiency of the application procedures. An increase in weight after treatment is an 
indication of new material deposited in the sample by the consolidation treatment. 

o Depth of Penetration: The visual evaluation of penetration depth of the proposed 
consolidant(s) is made by simply splitting the treated samples parallel to the direction of 
consolidant flow or examining them utilizing a Scanning Electron Microscope. Superficial 
penetration tends to contribute to further deterioration through the formation of a hardened 
surface crust. 

o Color Change: The visual evaluation of treated and untreated samples are made to 
determine whether color change and/or surface gloss are created by application of the 
consolidant. 

o Water Vapor Transmission: The water vapor transmission of treated and untreated 
masonry samples are compared to determine the reduction in water vapor transmission 
created by the consolidant 

o Compressive Strength / Tensile Strength: The measured compressive strength and 
tensile strength of treated and untreated masonry samples are compared to estimate the 
improvement in the mechanical properties of masonry following consolidation and to 
determine the consolidating abilities of the consolidant 

o Salt Crystallization: The resistance of consolidated samples to the effects of severe 
weathering and salt crystallization is evaluated by comparing the test results of treated and 

46 



untreated samples. Such information is used to assess the long-term effectiveness of the 
consolidation treatment 

o Accelerated Weathering Tests: The durability and ultraviolet stability of the 
consolidation treatment is determined by means of chamber accelerated weathering tests. 
Chamber for Accelerated Decay (CAD) cycles combine chemical attack, water and salt 
action, thermal effects, and solar radiation which are the causes of stone decay.^ 

The consolidants which show the most effective performance for the specific 
masonry samples may be proposed for evaluation under field conditions. 

2.5.2. FIELD TESTING PROGRAM: 

Upon the completion of the laboratory tests, it is necessary that a series of field tests 
be made with each of the prescribed consolidation treatments. Field trials do not provide 
much information in the short term about the preservative effect of the treatment; 
nevertheless, they are valuable for the practical experience they provide. 

The on-site test area should be representative of the more severely weathered areas 
and its condition before treatment should be carefully recorded. The size of the test area is 
suggested as approximately 10 or 20 square feet.^* The test area should be thoroughly 
cleaned of surface contaminants and allowed to dry prior to application to get adequate 
penetration of the consolidant. Manufacturer's recommendations are followed for 
application procedures and rates. After allowing the treatment to cure, core samples are 
taken from the test area to be submitted for laboratory evaluation which verifies the 
previous laboratory findings. The comparison of the treated test area to the adjacent 
untreated area provides an indication of the treatment's effectiveness. Furthermore, on-site 
testing indicates the rate of application, rate of consumption and exact application 
procedures to be used under field conditions, such as the number of applications and how 
the products are applied. 

47 



Laboratory and field testing programs are intended to provide some criteria for the 
evaluation of stone consolidants which will make their selection easier and more precise. 
However, standard test methods are lacking. Various proposed tests are still in the 
development stage and it is not yet certain that they will in fact provide realistic assessment 
of the treatment 

Recently, an ASTM (American Society for Testing and Materials) committee 
(ASTM Committee E-6 on Buildings ) is in the process of establishing a guideline for the 
selection and use of stone consolidants. There has been some controversy regarding these 
guidelines. A concern has been voiced by some architectural conservators indicating that 
these guidelines may mislead some contractors in a way that they may not seek proper 
professional advice. On the other hand, many conservators favor the development of 
guidelines which provide basic information about the proper use of consolidants, including 
the need to seek the specialized service of a conservator in both treatment choice and 
application. 

Laboratory and field tests must also be done for demonstrating effect of combined 
treatments (e.g. cleaning, consolidation and fills). 



48 



END NOTES: CHAPTER 2 



1. L. Schnabel, " Laboratory Assesment of the Barium Hydroxide-Urea Process for the 
Consolidation of Limestone and Marble, " Thesis for Master of Science Degree, Graduate 
School of Architecture, Planning and Preservation, Columbia University, 1988, p. 13. 

2. G. Torraca, " General Philosophy of Stone Conservation, " in L. Lazzarini and R. 
Pieper, eds.. The Deterioration and Conservation of Stone . Notes ftxjm the International 
Venetian Courses on Stone Restoration, undated, p. 248. 

3. J. R. Clifton, Stone Consolidating Materials-A Status Report . NBS Technical Note 
1118, National Bureau of Standarts ( Washington, D.C.: U.S. Government Printing 
Office, 1980 ), p. 8. 

4. J. R. Clifton, " Laboratory Evaluation of Stone Consolidants, " in Adhesives and 
Consolidants . Preprints of the Contributions to the Paris Congress, Sept.2-8, 1984 ( 
London: IIC, 1984 ), p. 153; S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry 
and Stone Conservation, " in Vth Internat ional Congress on Deterioration and Conservation 
of Stone . Volume 2, Lausanne, Sept. 25-27, 1985, p. 835. 

5. D. W. Boyer, " A Field and Laboratory Testing Program: Determining the Suitability 
of Deteriorated Masonries for Chemical Consolidation, " APT Bulletin XIX, 4 (1987), pp. 
46-47. 

6. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation," 
pp.835-836; J. R. Clifton, Stone Consolidating Materials-A Status Report, pp.8- 10; J. 
Ashley-Smith and H. Wilks, eds.. Adhesives and Coatings . Conservation Science 
Teaching Series, Book 3 ( London: Museums & Galleries Commission, 1987 ), pp. 123- 
127. 

7. J. R. Clifton, Stone Consolidating Materials-A Status Report, p. 10. 

8. G. Torraca, " General Philosophy of Stone Conservation," p. 249. 

9. J. R. Clifton, Stone Consolidating Materials-A Status Report , p. 12; J. R. Clifton " 
Laboratory Evaluation of Stone Consolidants, " p. 154. 

10. G.G.Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric Pollution. 
Cleaning. Consolidation and Protection. Materials Science Monographs, 1 1 ( New York: 
Elsevier Science Publishers B.V., 1983 ), p. 246; E. D. Witte, P. Huget, and P. Van Den 
Broeck, " A Comparative Study of Three Consolidation Methods on Limestone, " Studies 
in Conservation . 22, 4 (1977), p. 195; C. A. Price, " Brethane Stone Preservative, " p. 4. 

11. G. G. Amoroso and V. Fassina, Stone Decay and Conservation: Atmospheric 
Pollution. Cleaning. Consolidation and Protection, p. 244. 



49 



12. J. R. Clifton, Stone Consolidating Materials-A Status Report , p. 23; C. A. Price, 
"Brethane Stone Preservative," Building Research Establishment Current Paper . CP/81 
(Garston, England: Building Research Establishment, 1981), p. 3. 

13. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation," 
p. 831. 

14. Ibid., p. 833. 

15. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " Art and Archaeologv Technical Abstracts 18, 1 (1981), p. 152; 
C. A. Price, " Brethane Stone Preservative," p. 4. 

16. J. H. Larson, " A Museum Approach to the Techniques of Stone Conservation," in K. 
L. Gauri and J. A. Gwinn, eds.. Fourth International Congress on the Deterioration and 
Preservation of Stone Objects . Louisville, Kentucky, July 7-9, 1982 (Louisville: The 
University of Louisville, 1982), p. 232; C. A. Price, " Brethane Stone Preservative," p. 4. 

17. C. A. Price, " Brethane Stone Preservative," p. 4. 

18. C. S. Ewart, " Review of Stone Consolidants, " ICCROM, Rome, unpublished, 
1987, p. 11. 

19. C. V. Horie, Materials fo r Conservation: Organic Consolidants. Adhesives and 
Coatings ( London: Butterworth & Co. Publishers Ltd., 1987 ), p. 159. 

20. J. Riederer, " Recent Advances in Stone Conservation in Germany, " in E.M.Winkler 
,ed.. Decay and Preservation of Stone . Engineering Geology Case Histories, 1 1 ( Boulder, 
CO: The Geological Society of America, 1978 ), p. 92; J. R. Clifton, Stone Consolidating 
Materials-A Status Report, p. 24; C. A. Price, p. 4. 

21. N. R. Weiss, " Three Research Problems, " in International Symposium on the 
Conservation and Restoration of Cultural Propertv: The Conservation of Wooden Cultural 
property . Tokyo & Saitama, November 1-6, 1982, p. 289. 

22. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation," 
p. 833. 

23. A. E. Charola, " Brief Introduction to Silanes, Siloxanes, Silicones and Silicate 
Esters," in L. Lazzarini and R. Pieper, eds.. The Deterioration and Conservation of Stone. 
Notes from the International Venetian Courses on Stone Restoration, 1988, p. 313; C. A. 
Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," p. 153; C. A. Price, " Brethane Stone Preservative," p.l. 

24. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," p. 152; G.G. Amoroso and V.Fassina. Stone Decay and 
Conservation: Atmospheric Pollution. Cleaning . Consolidation and Protection, pp. 350- 
353. 

25. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " p. 152. 



50 



26. J. Ashley-Smith and H. Wilks, eds., Adhesives and Coatings , pp. 124-125. 

27. J. Riederer, " Recent Advances in Stone Conservation in Germany," p. 92; C. A. 
Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861- 1981," p. 152. 

28. G. Torraca, Porous Building Materials: Materials Science for Architectural 
Conservation . 3rd edition ( Rome: ICCROM, 1988 ), pp. 135-136; S. Z. Lewin, " The 
Current State of the Art in the Use of Synthetic Materials for Stone Conservation, Part 1. 
Inorganic and Metal-Organic Compounds," in L. Lazzarini and R. Pieper, eds.. The 
Deterioration and Conservation of Stone . Notes from the International Venetian Courses on 
Stone Restoration, 1988, p. 297. 

29. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," pp. 152-154. 

30. J. Ashley-Smith and H. Wilks, eds., Adhesives and Coatings , p. 125. 

31. Ibid. 

32. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation," 
p. 833. 

33. Ibid., p. 832. 

34. G. E. Wheeler, " The Chemistry of Four Alkoxysilanes and Their Potential for Use as 
Stone Consolidants," Dissertation for Doctor of Philosophy Degree, New York University, 
1987; S. Z. Lewin, " The Current State of the Art in the Use of Synthetic Materials for 
Stone Conservation, Part 1. Inorganic and Metal-Organic Compounds, " p. 295. 

35. A. E. Charola, G. E. Wheeler and G. G. Freund, " The Influence of Relative 
Humidity in the Polymerization of Methyl Trimethoxy Silane, " in Adhesives and 
Consolidants . Preprints of the Contributions to the Paris Congress, Sept. 2-8, 1984 ( 
London: IIC, 1984 ), p. 177; S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry 
and Stone Conservation," p. 836; G. E. Wheeler, " The Chemistry of Four Alkoxysilanes 
and Their Potential for Use as Stone Consolidants," pp. 21-26. 

36. A. E. Charola, G. E. Wheeler and G. G. Freund, " The Influence of Relative 
Humidity in the Polymerization of Methyl Trimethoxy Silane," P. 180. 

37. M. Roth, " Comparison of Silicone Resins, Siliconates, Silanes and Siloxanes as 
Water Repellent Treatment for Masonry," Technical Bulletin 983-1 ( Kansas City, Kansas: 
ProSoCo, Inc.), p.6; S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone 
Conservation, " p. 842. 

38. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation, " 
p. 842. 

39. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," P. 152. 

40. Ibid., p. 153. 

51 



41. J. R. Clifton. Stone Consolidaring Materials A Status Report p. 23. 

42. A. E. Charola, " Brief Introduction to Silanes, Siloxanes, Silicones and Silicate 
Esters, " p. 315. 

43. A. E. Charola, G. E. Wheeler and G. G. Freund, " The Influence of Relative 
Humidity in the Polymerization of Methyl Trimethoxy Silane," pp. 177-181. 

44. G. E. Wheeler, " The Chemistry of Four Alkoxysilanes and Their Potential for Use as 
Stone Consolidants," p. 4. 

45. A. E. Charola, G. E. Wheeler and G. G. Freund, " The Influence of Relative 
Humidity in the Polymerization of Methyl Trimethoxy Silane," pp. 177-181. 

46. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds," p. 296. 

47. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds," p. 296; G. Chiari, " 
Consolidation of Adobe with Ethyl Silicate: Control of Long Term Effects Using SEM," in 
Alejandro Alva and Hugo Houben, coordinators, 5th International Meeting of Experts on 
the Conservation of Earthen Architecture . Rome, Oct. 22-23, 1987 ( ICCROM / CRAterre, 
France, 1988 ), pp. 25-33. 

48. C. M. Paleos and E. G. Mavroyannakis, " Conservation of Ancient Terra Cotta 
Sherds by Alkoxysilanes, " in Preprints of 5th Triennial Meeting . Zagreb, Oct. 1-8, 1978, 
pp. 4-5; B. M. Feilden, Conservation of Historic Buildings . Technical Studies in the Arts, 
Archaeology and Architecture ( London: Butterworth & Co. Publishers Ltd., 1982 ), pp. 
342-343. 

49. H. Weber and K. Zinsmeister, Conservation of Natural Stone: Guidelines to 
Consolidation. Restoration and Preservation ( Ehningen bei Boblingen: expert - verlag, 
1991), p.115. 

50. S. Z. Lewin, " The Current State of the Art in the use of Synthetic Materials for Stone 
Conservation, Part 1. Inorganic and Metal-Organic Compounds," p. 297. 

51. ProSoCo, Inc., " H Stone Strengthener, Conservare, " and " OH Stone Strengthener, 
Conservare," Product Data ( Kansas City, Kansas: ProSoCo, Inc. ) 

52. G. A. Sleater, A Review of Natural Stone Preservation. NBSIR 74-444, National 
Bureau of Standards ( Washington, D.C.: U.S. Government Printing Office, 1973 ), p. 
17; G. A. Sleater, Stone Preservatives: Methods of Laboratory Testing and Preliminary 
Performance Criteria. NBS Technical Note 941, National Bureau of Standards ( 
Washington, D.C.: U.S. Government Printing Office, 1977 ), pp. 5-21; E. M. Winkler, " 
Testing Techniques for the Effectiveness of Stone Consolidants," APT Bulletin XVn, 2 ( 
1985 ), pp. 35-36. 

53. G. A. Sleater, A Review of Natural Stone Preservation. NBSIR 74-444, p. E-1. 

54. E. M. Winkler, " Testing for the Effectiveness of Stone Consolidants," pp. 35-37. 

52 



55. D. W. Boyer, " A Field and Laboratory Testing Program Determining the Suitability 
of Deteriorated Masonries for Chemical Consolidation," pp. 48-50; K. J. H. Zinsmeister, 
N. R. Weiss, and F.R.Gale, " Laboratory Evaluation of Consolidation Treatment of 
MassiUon ( Ohio ) Sandstone," APT Bulletin XX, 3 ( 1988 ), p. 36. 

56. D. W. Boyer, " A Field and Laboratory Testing Program Determining the Suitability 
of Deteriorated Masonries for Chemical Consolidation," pp. 50-51; K. J. H. Zinsmeister, 
N. R. Weiss, and F.R.Gale, " Laboratory Evaluation of Consolidation Treatment of 
MassiUon ( Ohio ) Sandstone," pp. 36-38; J. R. Clifton, " Laboratory Evaluation of Stone 
Consolidants," pp. 151-155; L. Arnold and C. A. Price, " The Laboratory Assesment of 
Stone Preservatives, " in R. Rossi-Manaresi, ed.. The Conservation of Stone L 
Proceedings of the International Symposium, Bologna, June 19-21, 1975 ( Bologna: 
Centro per la Conservazione delle Sculture all'Aperto, 1976 ), pp. 695-704. 

57. G. A. Sleater, Stone Preservatives: Methods of La boratory Testing and Preliminary 
Performance Criteria. NBS Technical Note 941, p. 8. 

58. D. W. Boyer, " A Field and Laboratory Testing Program Determining the Suitability 
of Deteriorated Masonries for Chemical Consolidation," p. 52; ProSoCo, Inc., Technical 
Bulletin 483-2. Conservare ( Kansas City, Kansas: ProSoCo, Inc.). 



53 



CHAPTER 3 
REVIEW OF USE OF ALKOXYSILANES IN CONSERVATION 

3.1. CONSOLIDATION OF STONE: 

3.1.1. HISTORY OF THE USE OF ALKOXYSILANES FOR STONE 

CONSOLIDATION: 

As consolidants, alkoxysilanes, especially ethyl silicate, have found a wide 
acceptance in present conservation practice. Current literature on consolidation of 
deteriorated stone contains numerous projects which have utilized ethyl silicates as 
consolidants with significant success. However, the idea of using ethyl silicate for 
conservation of stone is not new; it dates back to 1861. 

The synthesis of silicon tetrachloride, which was reported by J. J. Berzelius in 
1824, stimulated the synthesis of organosilicon compounds, among them tetraethoxysilane 
and triethoxymethylsilane.' In 1846, Von Ebelmen prepared silicic acid ether from silicon 
tetrachloride and alcohol. In 1874, triethoxymethylsilane was synthesized by A. 
Ladenberg. Until these substances became available as an industrial chemical, they 
remained a laboratory curiosity to conservators. 

The earliest attempt to use silicic ether or ethyl silicate for stone conservation was 
made in an 1861 meeting of the Royal Institute of British Architects by A. Hofmann for 
treatment of the Houses of Parliament in London.^ However, it is not clear if this subtance 
was ever utilized on the buildings until the 1920's. In 1923, A. P. Laurie obtained a British 
patent for the use of silicic ether in stone consolidation. Following this patent, he was 
granted three similar patents in the United States in 1925 and 1926.^ The liquid he 
suggested was actually a partially polymerized form of ethyl silicate. Afterwards, to some 

54 



extent, silicon esters became available as a commercial product especially in England. 
During the period from 1927 to 1939, hydrolised derivatives of ethyl silicate, which were 
broadly studied, interested chemists in a variety of industries, such as the manufacture of 
paints, coatings, fire-proof textiles, impregnating agents for preservation of stone, brick or 
woodwork, etc.'' 

In 1932, R. J. Schaffer published a report on the results of weathering behaviour of 
natural stone treated with silicon esters, sodium silicate, limewash, paraffin wax and other 
chemicals.' None of the preservatives was regarded as effective for the protection of stone. 
On the other hand in 1934, F. Rath gen and J. Koch mentioned silicic acid esters as 
effective agents for stone consolidation as a result of extensive experimentation with 
different stone protection agents.^ Another British patent obtained by G. King in 1939 
claimed the use of fungicides, insecticides, germicides, antiseptic or other substances with 
silicic esters mainly for masonry preservation.'' 

Around the time of World War II, the improvements relating to the organosilicon 
industry were rapid. Partially polymerized ethyl silicates, such as ethyl silicate 40, began to 
be widely marketed. Around 1947, some of the commercial products available from 
England through Silicon (Organic) Developments Ltd. were: Silester O, apparently ethyl 
silicate 40; Silester 1 and Silester 2, more highly polymerized ethyl silicate; Kexcement, a 
mixture of sillimanite and Silester 2.* Treatment of stone with ethyl silicate had its start in 
England, but as early as 1947, experimental studies with various chemical formulations of 
ethyl silicates, such as condensed ethyl silicate, ethyl silicate 40, were carried out at various 
historic sites in the United States: Chapel at Valley Forge, PA., the Eternal Light Peace 
Monument at Gettysburg, PA., adobe huts in Wyoming National Parks, etc' In the early 
1950's, the influence of catalysts on the hydrolysis of ethyl silicate was studied in detail.'" 
The first publication describing the use of ethyl silicate/methyl(trialkoxy)silane mixtures for 
stone preservation by J. Blasej et al. in 1959 opened a new era in stone consolidation. •• 

55 



3.1.2. RECENT EXPERIMENTS WITH ALKOXYSILANES IN STONE 
CONSOLIDATION: 

Development of alkoxysilane mixtures stimulated laboratory research and marketing 
of such products in Europe, particularly in Germany. During the 1960's, laboratory 
research and experimental field work with silicic acid esters as stone conservation agents 
were carried out extensively in Germany. Promising results of these tests led to the 
development of one and two-component stone consolidants based on pure silicic acid esters 
or a combination of silicic acid esters and organosilicon hydrophobic agents.'^ Around 
1970, German manufacturers such as Wacker-Chemie, Goldschmith AG, Bau-and Silikat 
Chemie began to market stone consolidants containing chemicals of these types. Two of the 
most frequently used products are Stone Strengthener H and Stone Strengthener OH which 
are marketed by Wacker-Chemie. These products are in the form of one-pack materials. In 
contrast to the Stone Strengthener OH, H product provides water repellency as well as 
consolidation. 

German literature contains numerous enthusiastic reports about the successful 
application of silicic acid based products to consolidate deteriorated German sandstones. 
Some of the historic structures treated with alkoxysilanes are Bamberg Cathedral (1973), 
Cologne Cathedral (1975), and the Alte Pinakothek in Munich (1975-1976). A partial list 

o 

of historically important structures and/or statuary which have been consolidated with 
alkoxysilanes over the years is presented in Appendix C. It includes both European and 
United States applications. 

Josef Riederer reported that during the early 1970's, progress was made not only 
in the field of research, but also in the techniques applied in Germany. '^ He emphasized 
that between 1972 and 1975 the use of ethyl silicates for stone consolidation increased to 
38% of all treatments (193 total) in Bavaria, while the use of other polymers and soluble 
silicates decreased. Riederer claimed that in the case of porous stones, ethyl silicate has 

56 



proved to be superior to all other stone consolidants. In his report, the depth of penetration 
in an ordinary sandstone with ethyl silicate is said to be about 4 cm. 

On the other hand, as of 1964 the overall British experience with ethyl silicates 
were reported dissapointing by Bailey and Schaffer.'"* In 1972, John Ashurst and Brian 
Clarke also reported that any of the stone preservatives, among them ethyl silicate, field 
tested since 1961 by the Building Research Station and the Ancient Monuments Branch of 
the Ministry of Works (now Department of the Environment) showed no overall beneficial 
effects. ^5 In spite of these negative results, the use of alkoxysilanes were reexamined by 
many researchers. Clifford Price, who was a Senior Scientific Officer at the Building 
Research Station in England, stated that alkoxysilanes were considered to be suitable 
materials for stone preservation, although they were listed as the most expensive stone 
preservatives in England*^ 

In 1976, a new consolidant, called Brethane, was developed by C. Price and his 
team at the Building Research Establishment ^^ Brethane is identified as a three-component 
product which is mixed immediately before use. The comf)onents of Brethane are 
trimethoxymethylsilane, ethanol, and water. It was reported by Price that, in addition to 
laboratory testing, field trials were carried out with Brethane involving the treatment of a 
wide variety of stonework in situ at twenty four sites between 1976 and 1979 to assess the 
effectiveness of the product. The product was reported to reduce the rate of stone decay 
very substantially. 

Price also pointed out the limitations of Brethane. Treatment was said to be 
irreversible, but reappUcation is possible. Some short term sponginess and darkening in the 
stone were reported. Brethane was said to be unsuitable for stonework which is heavily 
contaminated with sodium chloride, since this salt interferes with the setting of Brethane. 
The product was not recommended for use where there is rising damp. Additionally, high 
cost and consumption of the material to achieve deep impregnation was noted as a 

57 



drawback. It was estimated that 5 litres of Brethane per square metre is required to have a 
penetration of 25 mm. in a stone with 20% porosity. Additionaly, it was suggested that 
labor costs can be very high, because an operator may only be able to treat an area of 1 m^ 
per day. 

Further laboratory studies and field trials were suggested to make a decision about 
licencing the production and use of Brethane. Since Brethane was a newly developed 
consolidant, its use was recommended only for cases where decay was advanced. In 1983, 
Brethane began being marketed by Colebrand Ltd. and it is still on the market. 

The Victoria and Albert Museum in London has been actively involved in the 
problems of stone deterioration and its treatment since 1965. In his 1982 report, Larson 
outlined the Museum's approach to the techniques of stone conservation, particularly 
stressing the use of alkoxysilanes for consolidation of stone and marble.'^ Experiments 
with alkoxysilanes for stone consolidation began in the early 1970's at the Victoria and 
Albert Museum. Between 1970 and 1982, a variety of commercially available silanes, 
which included X54-802 (trimethoxymethylsilane), ICI EP 5850 (triethoxymethylsilane), 
Wacker VP 1301 (ethyl silicate and triethoxyethylsilane), and Tegovakon (a material similar 
to the Wacker VP 1301), were tested on a range of stones including Carrara marble, several 
English limestones and some English sandstones. 

The author noted that although they all provided some consolidation, gave good 
penetration and produced little color change on the treated surface, it was X54-802 that 
gave consistentiy good results. Anne Moncrieff, a Senior Scientific Officer at the Victoria 
and Albert Museum, reported that penetration of 5 cm. was achieved with this material on 
moderately weathered marble. •' However, the use of X54-802 was discontinued at the 
Museum in 1977 due to the several problems encountered during its use.^o Experimental 
woric with another product, Dow-Coming T.40149 (trimethoxymethylsilane), was initiated 
in 1976 at the Museum. As of 1982, J. H. Larson reported that excellent results were 

58 



obtained with this material over a period of six years.^' 

The penetration of the consolidant in depth is necessary to obtain a lasting effect of 
the treatment. Not only consolidant itself, but also the method of application has an 
influence on the effectiveness of a consolidation treatment. Several conservators have 
expressed their opinions regarding the improvements of application methods to attain 
deeper penetration with consolidants. 

R. Rossi-Manaresi suggested in her paper (1975) that deep penetration is obtained 
by keeping large quantities of consolidant in close and very long contact with the stone by 
dabbing with a brush or by a direct, slow percolation process on the surface.^ She added 
that application by spraying can not provide such a close contact. 

On the other hand, Rolf Wihr advocated the use of a spray system providing a 
continuous rain of consolidant and recirculating run off material in order to obtain a deeper 
penetration, based on observation that stone gradually becomes fully soaked in rain.23 
Although the high consumption of material and cost of spray apparatus were admitted as 
disadvantages, the author noted that this method is suitable for large scale building surfaces 
and provides a greater penetration depth than any other method. It was said that 800 liters 
of Wacker Stone Strengthener OH was applied by this method to the facade of the fortress 
of Wurzburg measuring 80 m^ in two and a half hours. A penetration depth of up to 25 cm. 
was achieved. The method was said to be under patent in most European countries. 

According to Helmut Weber, repeat application after one or two weeks is an 
effective method of consolidation treatment, as well as continuous spray applications.^" The 
repeat application has been extensively used on deteriorated sandstones in Germany. The 
restoration of the Alte Pinakothek, which is a historically important art gallery building in 
Munich, is worth mentioning because of the conservation approach which was applied.^^ 
In 1975, the conservation program was initiated through laboratory and field testing. 
During the following years, larger on-site test areas were treated and reguarly monitored to 

59 



evaluate the effectiveness of a variety of consolidation treatments by comparing treated and 
untreated areas. These tests revealed that a combinaton of silicic acid esters (Wacker Stone 
Strengthener OH) followed by silicic acid esters with water repellent additives (Wacker 
Stone Strengthener H) provided the most satisfactory results. After 10 years of evaluation, 
previous test results were confirmed and the overall conservation began in 1984. 

A vacuum system, named the Balvac system, was developed at the Victoria and 
Albert Museum for consolidation of decayed stone objects in 1975.2^ It was reported that 
with this method, the penetration of the consolidant was greater than that which could be 
achieved by brush application. However, based on the experiments with vacuum 
consolidation at the Museum, J. H. Larson pointed out his objections to the system by 
saying that there are many deficiencies in the use of vacuum impregnation techniques. He 
claimed that in some cases in England, severe cracking of stone sculpture occurred as a 
result of the vacuum method. In addition, he reported that, in six years time, two of the 
sculptures treated with vacuum consolidation at the Museum in 1976 deteriorated more 
rapidly than the objects treated with brush. He argued that the vacuum method does not 
seem to encapsulate salts more than brush applied systems. Another drawback reported 
was that after the vacuum is released, the consolidant tends to migrate inwards resulting in 
lesser consolidation of the surface than of the inside of the stone. Also, this method is of no 
use on buildings. 

Increasing research into the development of alkoxysilanes as stone consolidants led 
to development of mixtures of acrylics and alkoxysilanes as an alternative. It is believed 
that combining the qualities of both products might create a more effective range of 
consolidants. 

Acrylic/silane mixtures have been used extensively in the restoration of several 
monuments in Italy. Between 1973 and 1974, the Centre per la Conservazione delle 
Sculture AH'aperto carried out a series of experimental testing of ten commercially available 

60 



stone consolidants applied to a calcitic Bologna sandstone. ^^ The main concern of the study 
was to find efficient methods of consolidation for the sandstone monuments in Bologna 
which were in an advanced state of deterioration. Each consolidant was applied both in 
situ, on different monuments, and in the laboratory on samples of the same sandstone. 

As part of this study, O. Nonfarmale carried out the treatment of the Francisco 
Coriolani tomb (1555) and the Joannes Brand tomb (15th century) in the cloister of St. 
Domenico's church in Bologna.^^ Exfoliating and crumbling sandstone of these tombs 
were consolidated by a mixture of acrylic resin (Paraloid B-72) and silane (Dri-Film 104, 
prepolymerized methylalkoxysilane) in organic solvent, based on the thought that the 
adhesive properties of the acrylic resin and the consolidating, water repellent properties of 
the silane could be combined. The surface was impregnated with the mixture, first by 
spraying then by brushing (total 5 applications) to prevent the breaking away of raised 
crusts. The restoration was completed with a mechanical cleaning process. Overall, the 
treatment was reported as successful and long lasting. After treatment, all the raised crusts 
were reattached to the surface. 

However, in a later report R. Rossi-Manaresi and A. Tucci pointed out that the 
mixture Paraloid B-72/Dri-Film 104 has a disadvantage in that it causes a slight darkening 
of the treated stone.^' In addition, preconsolidation of exfoliating sandstone with this 
mixture was said to prevent subsequent cleaning by pack techniques, such as poultices. 
Because reattached crusts may be loosened and detached through the application of pack 
techniques, it seems that only mechanical cleaning is applicable in this case, which is time 
consuming and not feasible for large surfaces. The water repellent nature of the mixture 
also prevents cleaning with water. 

The authors proposed that a preconsolidation of similar calcite-cemented sandstone 
with an acrylic/siliconate/limewater mixture (Primal AC33/ Silirain- Water/ Limewater) 
appears to reattach loose flakes, does not produce any darkening effect and allows an easy, 

61 



safe cleaning utilizing pack techniques. They investigated the possibility of using this 
mixture, instead of Paraloid B-72/Dri-Film 104, for final impregnation, too. It was 
concluded that the more concentrated formulation of acrylic/siliconate/limewater mixture is 
adequate for final repeated impregnation, since it proved to provide a consolidative and 
protective effect similar to that of Paraloid B-72/Dri-Film 104. However, this treatment was 
reported to produce a whitening effect, if not properly applied. 

The Victoria and Albert Museum in London started experimental testing with 
acrylic/silane mixtures in 1978.^° J. H. Larson claimed that silanes make very good deep 
consolidants, but poor surface consolidants, whereas acrylics are known as good surface 
consolidants, but are not effective for consolidation in depth. Thus, the combination of two 
might exhibit better performance. 

One of the mixtures tested was Raccanello E.0057 (acrylic) diluted with Dow- 
Coming T.40149 (trimethoxymethylsilane) forming solutions varying from 5%-20% (by 
volume). This treatment is mentioned as the main consolidation treatment used at the 
Museum at the time (1982). The author indicated that although this combination produces 
only a mixture rather than a chemical bonding, it has several advantages over the majority 
of silane treatments. 

For one thing, extremely good penetration into stone and marble was achieved with 
this mixture simply by brush application. The penetration of 5 cm. was achieved on a 
French limestone sample with this treatment in the Museum. Retreatment with the above 
mixture was also said to be possible. It was said that preconsolidation carried out with the 
same mixture can allow subsequent cleaning by using solvents, such as acetone and 
toluene. Furthermore, the ease of bonding the pieces treated with acrylic/silane mixture by 
using a range of acrylic, polyester and epoxy adhesives and long-term durability of acrylic 
fillers for acrylic/silane consolidated stones were mentioned as advantages. 

Larson reported that two sculptures in the Museum (Bernini's 'Neptune and Triton' 

62 



Sculpture, Carrara marble; 12th century English Sculpture, Caen stone) were consolidated 
successfully with T.40149/E.(X)57 around 1982. 

In 1980, experimental testing with another acrylic/silane mixture, Dow-Coming 
T. 40149 and Paraloid B-72, started in the Victoria and Albert Museum. Paraloid B-72 was 
said to be dissolved in T.40149 without the addition of any solvent, so that the acrylic 
content of the solution could be quantified. The effectiveness of the treatment was reported 
to be very promising based on trials on English sandstones, limestones and Carrara marble 
with solutions containing between 2% and 5% of acrylic to silane. 

As a result of extensive practical experience at the British Museum, S. B. Hanna 
reported that highly successful results were obtained by using silane systems in the 
treatment of badly deteriorated limestones.^' The British Museum adopted two different 
silanes for further evaluation for the treatment of salt-contaminated, extensively deteriorated 
limestones in the early 1980's. Wacker Stone Strengthener OH and uncatalyzed 
acrylic/silane mixture (Dow-Coming T.40149/Raccanello E.55050) were chosen based on 
successful experiences elsewhere. 

Hanna indicated that although Wacker OH restores physical strength to the stone, it 
is more difficult to clean the surface after consolidation than when using the acrylic/silane 
mixture. After treatment with the Wacker system, the reattachment of detached flakes were 
said to be possible by using an acrylic resin. Retreatment is also claimed to be possible with 
this consolidant. 

According to Hanna, solutions of Raccanello E.55050 in Dow-Coming T.40149 
with varying strengths (5,10,15 and 20%-by volume) also yielded effective results. Initial 
darkening as a drawback of this treatment was reported. However, it was said to fade away 
during the following 12 months. The author advised not to apply more than 20% 
concentrations of acrylic, since percentages above this point can cause residual darkening 
and dismption within the stone if applied in more than 100 ml. quantities at any one time. It 

63 



appears that T.40149 and E.55050 do not form copolymers within the stone. It is assumed 
that the silane acts as a solvent carrying the acrylic to a greater depth of penetration than it 
would achieve on its own. 

In 1984, the badly degraded Egyptian limestone head of Amenophsis III (1417- 
1379 B.C.) was treated with both alkoxysilane and acrylic/silane mixture at the British 
Museum. 32 initial application of uncatalyzed silane (Dow-Coming T.40149, 
trimethoxymethylsilane) was said to aid the subsequent applications of the acrylic/silane 
mixture and increase the capacity of the stone to absorb more viscous acrylic/silane 
solution. During the subsequent apphcations the acrylic content of the solution was 
increased from 5% to 20%. Acetone was applied on cotton wool swabs to clean the surface 
after consolidation. 

Another report by S. M. Bradley regarding an investigation of the effectiveness of 
organo silanes on deteriorated limestone at the British Museum concluded that Wacker 
stone Strengthener H, Wacker Stone Strengthener OH and a solution of Raccanello acrylic 
in trimethoxymethylsilane are suitable materials for consolidation of deteriorated limestone 
sculpture displayed indoors.^^ A study of a variety of commercial stone consolidants on 
three different limestones by E. D. Witte et al. came to the conclusion that ethyl silicates are 
the nwst promising consolidants.^ 

It is apparent from the experiences reported above that alkoxysilanes are still 
promising materials for stone consolidation. A certain optimism has resulted fix)m increased 
research on the development of different techniques and the materials themselves. 
However, a lot more research needs to be done on the evaluation of the long-term 
effectiveness of past treatments as well as new materials, new combinations of materials 
and new techniques. 



64 



3.2. CONSOLIDATION OF EARTHEN BUILDING MATERIALS: 

Adobe, also called mudbrick or sun-dried brick, and related earthen building 
materials and techniques are among the oldest and most common building materials on 
earth. Constrtiction with rammed earth dates back to the Neolithic period (10,000 to 3,000 
B.C.).^^ Adobe is still widely used with various traditional techniques in many countries. 
Today, by various estimates, between 30 and 40 percent of the worid's population lives in 
earthen buildings. 

Adobe building materials are made by mixing sand, silt and clay with water to an 
adequate plasticity so that it can be formed in wooden molds or used as mortar or plaster. 
Often, fibrous organic materials, such as straw and animal hair are added to reduce the 
cracking when the adobe dries. Adobe, by its composition, is less durable than stone and 
obviously more susceptible to weathering. GeneraUy, the deterioration of adobe structures 
is attributable to excessive moisture, either rainwater or ground water. When the moisture 
content reaches an extreme level, the mechanical strength of the adobe is reduced beyond 
the loads placed on it As a result, the structure can collapse partially or completely.^ The 
consolidation of vertical surfaces and the capping of the top part of earth walls play an 
important role among the various preservation techniques of earthen architecture, especially 
earthen archaeological sites. Mainly, synthetic resins, usually thermoplastics, and ethyl 
silicates are used as consolidants on adobe. Since 1969, ethyl silicate has been applied to 
the consolidation of adobe with satisfactory results. 

As early as 1942, John B. Stone and Abraham J. Teplitz obtained a patent for the 
use of "alkyl silicate" in earth consolidation, with particular application to oil wells.^' 
Partially jwlymerized ethyl silicates were considered preferable in order to create high 
strength and hardness. It was not until 26 years later that a strong interest began at an 
international level in the preservation problems associated with earth structures and a critical 
need for the preservation of earthen architecture has been acknowledged by many countries 

65 



where the tradition of using earth as a building material exists. 

In 1968, research regarding the conservation of archaeological sites in unbaked 
earth was carried out in Iraq.^s After studying the actual causes of the mud-brick 
deterioration, laboratory tests were performed for surface protection. Among the various 
products available at the time, ethyl silicate seemed to give the best preliminary results. In 
1969, large scale field tests with partially polymerized ethyl silicate were done in the 
Seleucia area and in Hatra. Ethyl silicate was sprayed onto the vertical surfaces of the 
mudbrick walls. 

There are several publications on the evaluation of this work. Giorgio Torraca 
reported in 1970 that the penetration of the partially polymerized ethyl silicate [Silester OS 
(Monsanto)] was poor (10-15 mm).^' A later report by Gilbert Bultinck in the same year 
indicated that partially polymerized ethyl silicate formed a hard surface layer without 
adhesion to the mudbrick, based on examination during the previous field tests in Iraq. 
According to Giacomo Chiari, approximately in two years time, ethyl silicate treated walls 
were still in perfect condition compared to untreated walls, in the spring of 1971. Twenty 
years after the field applications, he evaluated the performance of the field treatments with 
ethyl silicate."*' Ethyl silicate treatment was not effective on the surfaces of walls severely 
affected by salt water, especially at the Seleucia Archives. On the other hand, ethyl silicate 
treatment seemed to be sufficient to provide the necessary water resistance in Hatra where 
the water table was much lower. It was reported that even very fine details were perfectiy 
preserved. It should be stressed, however, that the site was simply abandoned and no 
maintenance work has been done since 1971. It is apparent that the lack of maintenance 
over such a long period played a significant role in the failure related to the various 
conservation measures undertaken in this site. Without constant maintenance and repair, 
there is little hope in preserving mudbrick structures, regardless of the treatment 
performed. 

66 



While this research was being carried out in Iraq, two international symposia on 
mudbrick conservation were held in Yazd, in 1971 and 1976, by the Iranian National 
Committee of ICOMOS. In following years, many other meetings on the conservation of 
mudbrick and other raw earth materials were held for the purpose of establishing a 
methodology for the study of earthen structures and archaeological ruins, exchange of ideas 
and experience, and expanding current knowledge of appropriate techniques for the 
conservation of earthen architecture. These meetings are hsted in Appendix D. 

Ethyl silicate based consoUdants have been applied with success in many other sites 
and countries, as well. The treatment of some painted and unpainted adobe friezes with 
ethyl silicate in two archaeological sites in Peru, Garagay and Chan Chan, were sponsored 
by UNESCO (1975-1977)."' The mud friezes in Chan Chan exhibited hygroscopic salt 
deposition. These salts are deposited by winds carrying small droplets of sea water. Small 
holes in the friezes were filled with mud before the appUcation of ethyl silicate. Treatment 
of adobe friezes in Chan Chan was particularly successful, even though they were exposed 
to sea aerosol for 10 years and subjected to a severe flood in 1983."^ 

In the case of Garagay, problems were different, because the friezes have painted 
figures. Several applications of ethyl sihcate were performed by spraying to consoUdate the 
mudbrick support of the painting even before attempting to clean the surface. Applications 
were a few days apart In addition, a series of injections of an acryUc emulsion were made 
through the preexisting cracks to strengthen the inner part of the wall. Finally, an acrylic 
resin (Paraloid) was applied to fix the superficial layers of painting. Chiari indicated in his 
1980 report that the results of the adobe frieze treatment in Peru were very encouraging. He 
also pointed out that the use of mixed techniques has an advantage of overlapping the best 
properties of each product, for instance ethyl silicate performs well on the surface as a 
consolidant, while acrylic or polyvinylic resins work well as binding agents, when injected 
inside the wall. 

67 



The mudbrick support of an ancient wall painting (3500-3000 B.C.) from the site 
of Teleilat Ghassul, Jordan was treated with an ethyl silicate consolidant [Stone 
Strengthener H (Wacker)] in 1978 and 1979.''3 The painting was broken into many pieces 
and fragments. After strengthening the paint layer and gluing the small fragments, the 
major pieces were completely impregnated from the rear face through capillary absorption 
by placing the mudbrick surface in a shallow pool of a 50 % solution of Wacker Stone 
Strengthener H in toluene. It was reported that the requisite degree of strengthening effect 
was produced for the previously very friable mudbrick and this eased the handling and 
prevented the damage which could have occurred during reassembling and mounting the 
pieces to reconstitute the total mural. After the completion of treatment, the painting was 
installed in the Amman Museum. 

There are several studies related to the long-term effectiveness of an ethyl silicate 
based consolidant on mudbrick. Lewin and Schwartzbaum studied the untreated and treated 
specimens of the Teleilat Ghassul mural painting, which remained for future study, to 
determine the effect of four years of normal indoor aging on the composition of the 
consolidant and on the impregnated material.^'' They concluded that the hydrolysis 
continues to occur very slowly during long-term aging, because reactive ethoxy groups 
were still present in the treated material. Whether this slow increase in the degree of 
hydrolysis during aging can result in a loss of consolidating and strengthening effect is 
open to debate. If so, a retreatment with ethyl silicate to regain the required degree of 
consolidation might be a possibility, but only if the material has regained its original 
porosity. ^5 jn addition, it is known that as the hydrolysis reactions continue toward 
completion, a decrease in volume is produced. However, it is not yet known whether or 
not this shrinking can generate stress in the material in the long- term.''* 

More recendy, in 1987, Chiari studied the samples from an adobe brick coming 
from the Huaca de la Luna in Trujillo, Peru for the purpose of obtaining more data on the 

68 



long-term behavior of ethyl silicate treatment.'''' The samples were treated by complete 
impregnation with partially polymerized ethyl silicate [ Silester ZNS (Monsanto)] dispersed 
in alcohol. Following treatment, the samples were photographed under a Scanning Electron 
Microscope (SEM) at different time intervals, from one day to sixteen months, to observe 
the chemical interaction of ethyl silicate based consolidant with adobe at the microscopic 
level. These observations also confirmed the slowness of the hydrolysis reaction, although 
the results of this study are considered preliminary due to the short time range. In addition, 
it was observed that the material regained its porosity and general appearance, but kept the 
desired property of water resistance. Based on the fact that the structure of the material 
undergoes so little modification with ethyl silicate based treatment, Chiari suggests that 
reapplication of other consolidants can be considered, if needed. He does not recommend 
the ethyl silicate based consoUdants blended with water repellent organosilicon components 
(such as Wacker H, a mixture of methyltriethoxysilane and tetraethoxysilane in toluene 
solvent and catalyst) for an adobe treatment. This is because water repellency, both at 
vapor and liquid level, is not a desirable property for adobe conservation."** 

In 1985, the Museum of New Mexico started an adobe test wall program at Fort 
Selden State Monument, in Southern New Mexico, USA.^' Test walls were constructed to 
monitor the erosional rates of selected adobe preservation techniques in the hope that the 
experiments will provide information regarding the long-term preservation requirements of 
the historic walls at Fort Selden, as well as other earth buildings. This phase of the test wall 
project is scheduled for completion in 1995. Experiments included assessing the efficacy of 
chemically amended mud plaster, observing capillary rise in relation to the different wall 
bases, and evaluation of various capping techniques to protect the top part of the walls. 
The chemical substances were either sprayed on the wall surface, applied with a paint 
roller, or mixed in with the mud plaster. It was ref)orted by Taylor that preservation 
techniques employed on these test walls showed a varied range of erosional patterns after 

69 



three and a half years of exposure.^o Overall, it was observed that the chemically amended 
mixes have eroded much less than the spray and roll-on applications. Spalling occurred in 
the plastered surfaces of the sprayed panels. 

A collaborative program was established at Fort Selden by the Getty Conservation 
Institute (GCI) and the Museum of New Mexico State Monument early in 1988.^i More test 
walls were built in order to evaluate various preservation materials and techniques that do 
not change the wall's physical appearance, such as plastering, capping, etc. The 
experiments included: the application of consolidants, consolidant application techniques, 
drainage, shelter designs and materials, rebiuial techniques for archaeological sites, and 
some structural reinforcing material and methods. 

The consolidants used for the treatment of the adobe test walls were alkoxysilanes 
[Stone Strengthener H (ProSoCo) and Stone Strengthener OH (ProSoCo)] and isocyanates 
which exhibited most satisfactory results against deterioration by water and salts during the 
preliminary laboratory testing performed by the Getty Conservation Institute at the end of 
1986.52 Alkoxysilanes were applied in different amounts and solvent mixtures (ketone and 
mineral spirits) by spraying, brushing, multiple coating, and bulk infiltration. Bulk 
infiltration is a technique whereby holes are drilled into the wall and funnels are used to 
deliver consolidant to a substantial depth where impregnation is obtained. Bulk infiltration 
was used in combination with surface application of consolidant by brushing. After one 
month's curing, test walls were subjected to an accelerated weathering water spray system 
in order to evaluate materials and procedures in a short time period. Two water spray cycles 
per day were applied for two months, and later on walls were allowed to weather naturally. 

After two and a half years of weathering, an evaluation of this experiment by 
Charles Selwitz et al. indicated that, in general, a mixture of methyltriethoxysilane and ethyl 
silicate was found to be effective while ethyl silicate was not.'^ T^g test walls treated with 
Stone Strengthener H showed good consolidation without discoloration. However, the 

70 



same walls exhibited surface erosion. The better result was obtained with Stone 
Strengthener H in a mineral spirits solvent rather than the ketone solvent. The authors 
suggested that it was the presence of heavier hydrocarbons in the solvent which provided 
additional water hairier and repellency properties. 

Trials with alkoxysilanes on the historic adobe fit)m the 19th century adobe remains 
at Fort Selden were also carried out. Although the aged adobe took up the consolidants, it 
was not mechanically strengthened. Authors theorize that a slow dehydration takes place 
which takes the clay to a form that prevents the water-based curing mechanism from 
occuring. They suggest a rehydration of aged adobe before consolidation may be required. 

Different findings were reported by Hehni who studied mudbrick samples firom two 
archaeological sites in Egypt to determine the deterioration factors of adobe and to find out 
the suitable consolidants for their conservation. 5'* Samples were treated with 
tetraethoxysilane, trimethylmethoxysilane, and methylmethacrylate-buthylacrylate 
copolymer. Two applications were performed, a month apart, then samples were allowed 
to cure for one month before scanning electron microscope examination. Deterioration of 
this particular mudbrick was due to ground water infiltration, temperature fluctuation 
between day and night and wind abrasion. In addition, the author claims that the adobe's 
texture, which is formed of ill-sorted and loosely packed constituent grains of different 
sizes, accelerated the deterioration. It was concluded from the obtained data that ethyl 
silicate is the most suitable consolidant for the studied Egyptian adobe. Ethyl silicate 
succeeded in forming the network links of the polymer, whereas trimethylmethoxysilane 
formed a less continuous layer on the grains. 

The composition and grain size distribution of adobe are important factors when 
choosing the right type of consolidant. Neville Agnew et al, studied the interaction of 
chemical consolidants with adobe and adobe constituents and came to the conclusion that 
adobes containing kaolinite are effectively consolidated with silanes.^^ On the other hand, 

71 



silanes do not develop a strong enough bond to bridge quartz and montmorillonite 
particles, which is why they do not perform well on adobes containing montmorillonite, 
and/or mixed-layer clays. 

As is obvious from the preceding discussion, controversial results have been 
obtained from different research and experiments regarding the use of alkoxysilanes for the 
conservation of earthen building materials. For instance, according to Chiari,^^ a suitable 
consolidant for adobe should provide water resistance but not water repellency in order to 
allow water migration both in liquid and vapor phase while the research project at Fort 
Selden confirms the effectiveness of alkoxysilanes which also provide water repellency. 
Although alkoxysilanes seem to be most promising consolidation materials for adobe, as in 
the case of stone consolidation, there is still a need for more research both on the materials 
and the application techniques. It is also crucial that past treatments continue to be 
monitored since this will yield very valuable information for researchers. 



72 



END NOTES; CHAPTER 3 



1. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981." Art and Archaeology Technical Abstracts 18. 1 (1981), pp. 150- 
151. 

2. Ibid., pp. 155-156. 

3. Ibid., pp. 156-157. 

4. Ibid., pp. 157-162. 

5. Ibid., pp.162. 

6. H. Weber, " Conservation and Restoration of Natural Stone in Europe" APT Bulletin 
XVn, 2 (1985), p.l5. 

7. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," pp. 162-163. 

8. Ibid., p. 166. 

9. Ibid., p. 165. 

10. Ibid., p. 151. 

11. Ibid., p. 171. 

12. H. Weber, " Conservation and Restoration of Natural Stone in Europe, " APT Bulletin 
XVn, 2 (1985), pp. 15-17; H. Weber, "Stone Renovation and Consolidation Using 
Silicones and Silicic Esters," in R. Rossi-Manaresi, ed.. The Conservation of Stone I . 
Proceedings of the International Symposium, Bologna, June 19-21, 1975 (Bologna: 
Centro per la Conservazione delle Sculture all'Apeno, 1976), pp. 380-381. 

13. J. Riederer, "Further Progress in German Stone Conservation," in The Conservation 
of Stone I . pp. 369-385; J. Riederer, " Recent Advances in Stone Conservation in 
Germany, " in E. M. Winkler, ed.. Decay and Preservation of Stone . Engineering 
Geology Case Histories, 1 1 (Boulder, CO: The Geological Society of America, 1978), pp. 
89-93. 

14. S. Z. Lewin and G. E. Wheeler, " Alkoxysilane Chemistry and Stone Conservation, " 
in Vth International Congress on Deterioration and Conservation of Stone . Volume 2, 
Lausanne, Sept. 25-27, 1985, p. 831. 

15. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " Art and Archaeology Technical Abstracts 18. 1 (1981), p. 177. 

73 



16. Ibid., p. 184. 

17. C. A. Price, "Brethane Stone Preservative," Building Research Establishment Current 
Paper . CP/81 (Garston, England: Building Research Establishment, 1981), pp. 1-9. 

18. J. H. Larson, "A Museum Approach to the Techniques of Stone Conservation," in K. 
L. Gauri and J. A. Gwinn, eds.. Fourth International Congress on the Deterioration and 
Preservation of Stone Objects . Louisville, Kentucky, July 7-9, 1982 (Louisville: The 
University of Louisville, 1982), pp. 219-237. 

19. A. Moncrieff, "The Treatment of Deteriorating Stone with Silicone Resins: Interim 
Report." Studies in Conservation 21.4 (1976), p. 181, 

20. J. H. Larson, "A Museum Approach to the Techniques of Stone Conservation," pp. 
224-227. 

21. Ibid., p. 227. 

22. R. Rossi-Manaresi, "Treatments for Sandstone Consolidation," in The Conservation of 
Stone I . p. 569. 

23. R. Wihr, "Deep-Impregnation for Effective Stone-Protection," in The Conservation of 
Stong I. pp. 317-318. 

24. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981," p. 192. 

25. H. Weber, " Conservation and Restoration of Natural Stone in Europe," pp. 17-23; H. 
Weber and K. Zinsmeister, Conservation of Natural Stone: Guidelines to Consolidation. 
Restoration and Preservation (Ehningen bei Boblingen: expert - verlag, 1991), pp. 151- 
162. 

26. J. H. Larson, "A Museum Approach to the Techniques of Stone Conservation," in 
Fourth International Congress on the Deteriorati on and Preservation of Stone Objects , pp. 
228-231. 

27. R. Rossi-Manaresi, "Treatments for Sandstone Consolidation," in The Conservation of 
Stone I . pp. 547-571. 

28. O. Nonfarmale, "A Method of Consolidation and Restoration for Decayed 
Sandstones," in The Conservation of Stone I . pp. 401-410. 

29. R. Rossi-Manaresi and A. Tucci, "The Treatment of Calcite-Cemented Sandstone with 
Acrylic-Siliconate Mixture in Limewater," in Adhesives and Consolidants . Preprints of the 
Contributions to the Paris Congress, Sept. 2-8, 1984 (London: IIC, 1984), pp. 163-166. 

30. J. H. Larson, "A Museum Approach to the Techniques of Stone Conservation," pp. 
231-237. 

31. S. B. Hanna, "THe Use of Organo-Silanes for the Treatment of Limestone in an 
Advanced State of Deterioration," in Adhesives and Consolidants . pp. 171-176. 

74 



32. Ibid., pp. 174-176. 

33. S. M. Bradley, "Evaluation of Organo Silanes for Use in Consolidation of Sculpture 
Diplayed Indoors," in Vth International Congress on Deterioration and Conservation of 
Stone . Volume 2, pp. 759-767. 

34. E. De Witte, A. E. Charola and R. P. Sherryl, "Preliminary Tests on Commercial 
Stone Consolidants," in Vth International Congress on Deterioration and Conservation of 
Stone. Volume 2, pp. 709-719. 

35. J. R. Clifton, Preservation of Historic Adobe Structures - A Status Report . NBS 
Technical Note 934, National Bureau of Standarts (Washington, D.C.: U.S. Government 
Printing Office, 1977), p. 2. 

36. A. Crosby, " The Causes and Effects of Decay on Adobe Structures," in Alejandro 
Alva and Hugo Houben, coordinators, 5th International Meeting of Experts on the 
Conservation of Earthen Architecture . Rome, Oct. 22-23, 1987 (Rome: 
ICCROM/CRATerre, 1988), pp. 33-41. 

37. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " Art and Archaeologv Technical Abstracts 18, 1 (1981), p. 163. 

38. G. Chiari, " Chemical Surface Treatments and Capping Techniques of Earthen 
Structures: A Long-Term Evaluation," in 6th International Conference on the Conservation 
of Earthen Architecture . Adobe 90 Pteprints, Las Cruces, New Mexico, USA, Oct. 14-19, 
1990 ( Los Angeles: Getty Conservation Institute, 1990 ), pp. 267-273. 

39. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " p. 175. 

40. G. Chiari, " Chemical Surface Treatments and Capping Techniques of Earthen 
Structures: A Long-Term Evaluation," pp. 267-273. 

41. G. Chiari, " Treatment of Adobe Friezes in Peru," in Third International Symposium 
on Mudbrick (Adobe) Preservation . Ankara (Turkey), Sept. 29-Oct. 4, 1980 (Ankara: 
ICOM-ICOMOS, 1980), pp. 39-45. 

42. G. Chiari, " Consolidation of Adobe with Ethyl Silicate: Control of Long Term Effects 
Using Sem," in Alejandro Alva and Hugo Houben, coordinators, 5th International Meeting 
of Experts on the Conservation of Earthen Architecture . Rome, Oct. 22-23, 1987 (Rome: 
ICCROM/CRATerre, 1988), p. 26. 

43. C. A. Grissom and N. R. Weiss, eds., " Alkoxysilanes in the Conservation of Art and 
Architecture: 1861-1981, " p. 196; S. Z. Lewin and P. M. Schwartzbaum, " Investigation 
of the Long-Term Effectiveness of an Ethyl Silicate-Based Consolidant on Mudbrick," in 
Adobe International Symposium and Training Workshop on the Conservation of Adobe. 
Lima-Cusco (Peru), Sept. 10-22, 1983 (Lima: ICCROM/UNDP/UNESCO Regional 
Project on Cultural Heritage and Development, 1984), p. 77. 

44. Ibid., pp. 77-81. 

75 



45. G. Chiari, " Consolidation of Adobe with Ethyl Silicate: Control of Long Term Effects 
Using Sem," in 5th International Meeting of Experts on the Conservation of Earthen 
Architecture , p. 26. 

46. S. Z. Lewin and P. M. Schwartzbaum, " Investigation of the Long-Term Effectiveness 
of an Ethyl Silicate-Based Consolidant on Mudbrick," in Adobe International Symposium 
and Training Workshop on the Conservation of Adobe . p.81. 

47. G. Chiari, " Consolidation of Adobe with Ethyl Silicate: Control of Long Term Effects 
Using Sem," in 5th International Meeting of Experts on the Conservation of Earthen 
Architecture, p. 25-32. 

48. G. Chiari, " Chemical Surface Treatments and Capping Techniques of Earthen 
Structures: A Long-Term Evaluation," p. 269. 

49. M. R. Taylor, " Fort Selden Test Wall Status Report," in 5th International Meeting of 
Experts on the Conservation of Earthen Architecture , pp. 91-102. 

50. M. R. Taylor, " The Fort Selden Adobe Test Wall Project," APT Bulletin XXn, 3 
(1990), pp. 35-41. 

51. N. Agnew, " The Getty Adobe Research Project at Fort Selden.I. Experimental Design 
for a Test Wall Project," in 6th International Conference on the Conservation of Earthen 
Architecture, pp. 243-249. 

52. R. Coffman, C. Selwitz and N. Agnew, " The Getty Adobe Research Project at Fort 
Selden. II. A Study of the Interaction of Chemical Consolidants with Adobe and Adobe 
Constituents," in 6th International Conference on the Conservation of Earthen Architecture . 
pp. 250-254. 

53. C. Selwitz, R. Coffman and N. Agnew, " The Getty Adobe Research Project at Fort 
Selden. in. An Evaluation of the Application of Chemical Consolidants to Test Walls, " in 
6th International Conference on the Conservation of Earthen Architecture , pp. 255-260. 

54. F. Helmi, " Deterioration and Conservation of Some Mud Brick in Egypt," in 6th 
International Conference on the Conservation of Earthen Architecture, pp. 277-282. 

55. R. Coffman, C. Selwitz and N. Agnew, " The Getty Adobe Research Project at Fort 
Selden. II. A Study of the Interaction of Chemical Consolidants with Adobe and Adobe 
Constituents," in 6th International Conference on the Conservation of Earthen Architecture. 
pp. 253. 

56. G. Chiari, " Chemical Surface Treatments and Capping Techniques of Earthen 
Structures: A Long-Term Evaluation," p. 267. 



76 



CHAPTER 4 
CONCLUSIONS AND RECOMMENDATIONS 

Off all products applied over the years, alkoxysilanes and related compounds are 
still the most promising consolidants. After more than twenty years of experience, 
alkoxysilanes have so far proved to be effective for the consolidation of sandstones, 
limestones and earthen building materials. Considering the chemical structure, their big 
advantage is that they polymerize to produce a chemically stable end-product similar to the 
minerals composing the stone (e.g. sandstone and earth) itself. Their ability to penetrate 
deeply into porous stone, which derives at least in part from the extremely low viscosity of 
the monomers, also makes alkoxysilanes attractive consolidants. 

While these properties favor the choice of alkoxysilanes as consolidants where such 
treatment is necessary, there are various factors which have not been fully explored. The 
mechanism of polymerization and the effect which the rates of hydrolysis and condensation 
reactions have on the structure and stability of the resulting polymer are some of the aspects 
that are still open to debate. Similarly, the effects of factors such as temperature, relative 
humidity and the presence of salts on the consolidation process are not completely 
understood. It is apparent that a lot more research needs to be done before a complete 
understanding of the functioning of silane systems is achieved. Not only the materials 
themselves and the conditions that affect the behavior of the materials, but also the 
application techniques utilized require further study. Controversy in the publications over 
the use of alkoxysilanes on stone and adobe conservation might partially result from the 
lack of research and an incomplete understanding of the complexity of alkoxysilane 
chemistry. 

77 



Other factors also need to be considered. For example, alkoxysilanes are rather 
expensive materials. The labor costs of application may also be very high. It would, 
therefore, be unreasonable to treat large areas with alkoxysilanes. Spray application also 
limits their use, making them more suited to small objects or architectural details rather than 
large building surfaces. 

There is no doubt that alkoxysilanes have extended the life of decaying stonework 
in many cases. However, alkoxysilanes, like other conservation materials, can provide a 
consolidative and protective effect only for a limited time period. Thereafter, reapplication 
of the same consolidant or a different one will become an issue. Although it has been 
claimed by many authors that alkoxysilane treated stone would be capable of absorbing a 
further treatment, very litde has been published about the retreatment of alkoxysilane treated 
stone or adobe. Many questions can be raised such as: Can alkoxysilane treated stone be 
retreated with the same or another consolidant? What will be the effect of retreatment on 
the first treatment? What will be the cumulative effects of repeated treatment on the original 
historic material? There is clearly a need for more research on the retreatment of 
consolidated masonry and earthen materials in order to provide answers to these and related 
questions. 

Current literature includes an increasing number of case studies where 
alkoxysilanes have been used as consolidants. However, information concerning the long- 
term effectiveness of these reported applications are rarely documented. Well-documented, 
detailed descriptions of the treatments and records of subsequent inspections including 
regular monitoring of the consolidated area should become part of the conservation 
program. This information will assist conservators in evaluating the effectiveness of 
various treatments. In this regard, documentation of unsuccessful consolidation is as 
valuable as successful work, because it too, provides important feedback. Documentation 
methods, which best record before and after treatment conditions, need to be further 

78 



investigated and standardized. 

However, although the long-term performance of a consolidant can only be proved 
with time, it is not always possible to wait for years in order to evaluate a particular 
treatment. In addition, lack of performance data for the newly developed materials creates a 
need for laboratory and field tests to provide a reliable indication of a treatment's 
effectiveness. Unfortunately, the development of such tests is still in a very early stage and 
it is not certain that they provide a realistic basis for the evaluation of a treatment. More 
work is needed to develop truly indicative laboratory and field tests for the evaluation of 
consolidation treatments. 

In conclusion, it is evident that there have been radical changes in the conservation 
of stone and earthen building materials in the past twenty years and these developments will 
undoubtedly be subject to further changes in the futiu-e. There will never be an 'ideal' 
material in the field of stone and adobe consolidation, because one can not expect to 
formulate one single system for a wide range of problems. Different projects will require 
different approaches and methods of treatment. Basic principles like minimum intervention 
and reversibility, or at least retreatability, must still be considered. Though research into 
new conservation materials has its place, it is only through a complete understanding of 
traditional materials and technologies and mechanisms of decay that suitable approaches can 
be developed for a wide variety of issues in conservation. 



79 



APPENDIX A 
COMMERCIALLY AVAILABLE ALKOXYSILANE CONSOLIDANTS 

This information is obtained from a variety of sources. The name of the products and the 
addresses of the manufacturers included in this list thus may be out-of date. 



PRODUCT NAME 



COMPONENTS 



MANUFACTURER 



MONUMENTIQUE 



MONUMENTIQUE 
Sandstone Consolidant 

Brethane 



One-component 

Sihcon Ester 

(without water repellent) 



Three-compound product 
Trimethoxymethylsilane, 
Ethanol, Water, and 
Manosec Lead-36 catalyst 



Bau-Chemie Prochaska und 
Pucher KG 



Bau-Chemie Prochaska und 
Pucher KG 



Colebrand Ltd. 



T4-0149 


Trimethoxymethylsilane 


Dow Coming Ltd. 


Z-6070 


Trimethoxymethylsilane 
(same chemical compound 
as T4-0149, but with 


Dow Coming Ltd. 




different marketing purposes) 


XI -90 10 


Triethoxysilane 


Dow Coming Ltd. 


Dynasil A 


Ethyl Sihcate 


Dynamit Nobel AG 


Dynasil 51 


Partially Polymerized 
Methyl Sihcate 


Dynamit Nobel AG 


Dynasil 40 


Partially Polymerized 
Ethyl Sihcate 


Dynamit Nobel AG 


DRI FILM 104 


Prepolymerized 
Methylalkoxysilane 


General Electric 



80 



PRODUCT NAME 


COMPONENTS 


MANUFACIURER 


TEGOVAKON H 


Two-component mixture of 
Silicon Esters with Water- 
Repellent 


Th. Goldschmidt AG 


TEGOVAKON OS 


One-component product 
similar to TEGOVAKON H 


Th. Goldschmidt AG 


TEGOVAKON V 


Ethyl Silicate 


Th. Goldschmidt AG 


TEGOVAKON T 


Ethyl Silicate and 
Triethoxymethylsilane 


Th. Goldschmidt AG 


Aqua-Trcte 


Alkylalkoxysilane(97%) 
Methanol 


Hills America, Inc. 


EP 5850 


Triethoxysilane 


ICI Organic EHvision 


SilesterOS 


Partially Polymerized 
Ethyl Silicate with 
40% available silica 


Monsanto Company 


SUesterZNS 


Partially Polymerized 
Ethyl Silicate with 
40% available silica 


Monsanto Company 


BR 74 


Partially Polymerized 


Montecatini, Society 


RaccaneUo E55050 


Acrylic-silane 


RaccaneUo 


RaccaneUoE0057 


Acrylic-silane 


RaccaneUo 


Wykester 


Silicon Ester 


Richardson and Starling, Ltd. 


Rhodorsil X54-802 


Trimethoxymethylsilane 


Rhone-Poulenc 


SilesterO 


Partially Polymerized 
Ethyl Silicate with 
40% available silica 


SUicon (Organic) 
Developments Ltd. 


Silester 1 


Partially Polymerized 
Ethyl Silicate with 
a condensing agent 


SUicon (Organic) 
Developments Ltd. 


Silester 2 


Partially Polymerized 
Ethyl Silicate with 
a condensing agent 


SUicon (Organic) 
Developments Ltd. 


Tetra(ethyl)orthosilicate 
(lEOS) 


Ethyl Silicate 


Union Carbide and Carbon 
Chemicals Corporation 



81 



PRODUCT NAME 



COMPONENTS 



MANUFACTURER 



Ethyl Silicate 40 



A-174 



A-llOO 



Strengthener H 
(Conservare H) 



Strengthener OH 
(Conservare OH) 



Conservare H40 Water 
Repellent 



Partially Polymerized 
Ethyl Silicate with 
40% available silica 

Methyl Acrylopropyl 
Trimethoxysilane 

Aminopropyl 
Trimethoxysilane 

One-component product 
Ethyl Silicate and 
Triethoxymethylsilane 

One-component product 
Ethyl Sihcate 



Union Carbide and Carbon 
Chemicals Corporation 

Union Carbide and Carbon 
Chemicals Corporation 

Union Carbide and Carbon 
Chemicals Corporation 

Wacker-Chemie GmbH 
Germany 
ProSoCo, IncAJSA 

Wacker-Chemie GmbH 
Germany 
ProSoCo, Inc. 

ProSoCo, Inc. 



MANUFACTURERS LIST: 



Bau-Chemie Prochaska und Pucher KG 
Ciba-Geigy AC 
Colebrand Ltd. 

Dow Coming, Ltd. 

Dynamit Nobel AG 
Th. Goldschmidt AG 
ICI Organic Division 



82 



81 Garmisch-Partenkirchen 
Austria 

CH-4002 Basel 
Switzerland 

20 Warwick St., Regent St. 
London, WIR 6BE 
England 

Midland 

Michigan, 48640, USA/ 

Barry, Glamorgam CF 677L 

England 

Troisdorf 
Germany 

Essen 
Germany 

Penart Research Center 
Penart House, Otterboume Hill 
Winchester, Hampshire, England 



MANUFACTURERS LIST: 



Monsanto Company 

Montecatini, Societk 
ProSoCo, Inc. 

Raccanello 

Rhone-Poulenc Industries 

Richardson and Starling, Ltd. 
Rohm GmbH 

Silicon(C)rganic) Developments, Ltd. 

Union Carbide Corp. 

Wacker-Chemie GmbH 



800 North Lindberg Blvd. 
St. Louis, MO 63166 
USA 

ViaF. Turatil8, 
Milan, Italy 

P.O.Box 1578 
Kansas City, KS 661 17 
USA 

Ard.F.Ui. Raccanello S.p.A. 
Industria Vemice e Smalti, Padua 
Italy 

22 Avenue Montaigne F 

75008 Paris 

France 

Winchester 
England 

Postfach 4242, Kirschenallee 
D6100 Darmstadt 1 
Germany 

1 1 Cavendish Place 
London Wl 
England 

30 E. 42nd Street, 
New York 
USA 

Sparte E, Postfach D800 
Munchen 22 
Germany 



83 



APPENDIX B 
TEST METHODS FOR THE LABORATORY EVALUATION OF STONE 

The test methods regarding natural stone listed below are taken from tests published by the 
American Society for Testing and Materials (ASTM) and the Deutsche Industrie Norms 
(German Industrial Standards - DIN). 

AMERICAN SOCIETY FOR TESTING AND MATERIALS STANDARD TEST 

METHODS REGARDING STONE: 

ASTM TEST METHOD PROPERTY 



B-1 17-64 Standard Method of Salt Spray (Fog) 

Testing. 

C-20 Apparent Porosity. 

C-88-69 Sodium Sulfate Crystallization. 

C-97-47 Absorption and Bulk Specific Gravity 

of Natural Bulding Stone. 

C-99-52 — — Modulus of Rupture. 

C- 170-50 Compressive Strength of Natural 

Building Stone. 

C-241-51 Abrasion Resistance. 

C-355-64 Standard Methods of Test for Water 

Vaf>or Transmisssion of Thick 
Materials (It can be used for stone). 

C-568-67 Standard Specification for Dimension 

Limestone. 

C-615-68 — Standard Specification for Structural 

Granite. 

84 



ASTM TEST METHOD PROPERTY 



C-616-68 Standard Specification for Building 

Sandstone. 

D- 119-71 Standard Definitions Relating to 

Natural Building Stones. 

D-523-67 — Standard Method of Test for Specular 

Gloss. 

D-2244-68 Instrumental Evaluation of Color 

Differences of Opaque. 

D-2247-68 Standard Test Method of Testing 

Coated Metal Specimens at 100 Percent 
Relative Humidity 

(Water Condensation / Evaporation). 
It can be used for stone. 

D-822 Recommended Practice for Operating 

Light- and Water- Exposure 
Apparatus (Carbon Arc Type). 

E- 18-67 Standaid Test Method for RockweU 

Hardness and Rockwell Superficial 
Hardness of Metallic Materials 
(Surface Hardness). 

E-96 Standard Test Metiiod for Water Vapor 

Transmission of Materials. 

G-27-70 Standard Recommended Practice for 

Operating Xenon-Arc Type Apparatus 
for Light Exposure of Nonmetdlic 
Materials. 



DEUTSCHE INDUSTRIE NORMS (GERMAN INDUSTRIAL STANDARDS-DIN) 

REGARDING STONE: 

DIN TEST METHOD PROPERTY 



50017 Testing of Materials, Structural 

Components and Equipment, Method 
of Test in Damp Heat Alternating 
Atmosphere Containing Sulfur Dioxide 



85 



DIN TEST METHOD 



PROPERTY 



50018 Sulfurous Acid Fog. 

52100 Guidelines for the Testing of Natural 

Stone. 

52101 — Sampling. 

52102 - Determination of Density, Bulk 

Density, True Density, Density 
* Grade, True Porosity. 

52103 Determination of Water Absoption. 

52105 Compression Test. 

52 1 06 Principles for the Assesment of the 

Weathering Resistance. 

52107 Resistance to Impact (cubes). 

52108 Wear Test with Grinding Wheel. 

52 1 1 1 Sodium Sulfate Crystallization Test. 

52112 HexureTest. 

52113 Determination of Saturation Value. 

52615 Water Vapor Transmission. 



ASTM Standards can be obtained from 



American Society for Testing and Materials (ASTM) 

1916 Race Street 

Philadelphia, PA. 19103-1187 

USA 

(215) 299-5585 



DIN Test Methods can be obtained from : 

Beuth-Vertrieb Gmb 
1 BerUn 30 

Burggrafenstrasse 4-7 
GERMANY 

Information about RJLEM was not available. 



86 



APPENDIX C 
SELECTED PROJECTS TREATED WITH ALKOXYSILANES 

The following is a partial list of selected buildings and/or statuary on which alkoxysilanes 
have been applied. The list includes both European and United States applications. This 
information is obtained from a variety of sources, including ProSoCo's " Conservare Stone 
Strcngtheners Reference List." 

EUROPEAN APPLICATIONS: 



Church of St. Francis 
Alghero, Sardina, Italy, 1973 

Bamberg Cathedral 
Bamberg, Germany, 1973 

Catholic Church 
Seeburg, Germany, 1973 

Church St. Aegidien 
Braunschewig, Germany, 1974 

Friedensangel (Angel of Peace) 
Munich, Germany, 1978 



Cologne Cathedral 
Cologne, Germany, 1975 

Apollo Temple 
Munich, Germany, 1975 

Town Hall 

Titmoning, Bavaria, Germany, 1975 

City Hall 

Zurich, Switzeriand, 1975 



Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener H 
Sandstone 

Wacker Stone Strengthener OH 



Wacker Stone Strengthener H 



Wacker Stone Strengthener OH 

andH 
Limestone 

Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 



87 



Town Hall 

Paderbom, Germany, 1975 

Old Picture Gallery 

Munich, Germany, 1975/1976 

Bavarian Portal 
Group of Statues 
Landsberg, Germany, 1976 



Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 
Sandstone 



Castle "Friedrichstein" 

Bad Wildungen, Germany, 1976 

Cathedral 

Statue "Man of Sorrow" 

Ulm, Germany, 1976 

Rathausen (Town Hall) 
Zurich, Switzerland, 1976 

"Bankgesellschaft" 

Zurich, Switzerland, Appx. 1976 

Tintem Abbey 
Nave column 
England, 1976 

Audley End 

George ni temple column 

England, 1976 

Fumess Abbey 
Chapter House roundel 
England, 1976 

Windsor Castle 
Norman Gate boss 
England, 1976 

Ministry of Finance 

Hanover, Germany, 1976/1977 

Overseas Museum 
Bremen, Germany, 1977 

Post Office 

Verden, Germany, 1977 

St. Martin's Church 
Kaiserslautem, Germany, 1977 



Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 
Sandstone 



Wacker Stone Strengthener OH 
Sandstone/Limestone 

Wacker Stone Strengthener OH 
Sandstone 

Brethane 
Sandstone 



Brethane 
Limestone 



Brethane 
Sandstone 



Brethane 
Calcareous sandstone 



Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 



88 



Town Hall 

Illertissen, Germany, 1977 

Tower of London 

Wakefield Tower (single block) 

England, 1977 

Goodrich Castle, Solar column 
England, 1977 

Wells Cathedral, Statue 1 17 
England, 1977 

King's Manor, Doorway 
York, England, 1977 

Sl Laurenzen Church 

St. Gallen, Switzerland, 1978 

St. Thomas Church 
Augsburg, Germany, 1978 

Roman Aqueduct 
Mainz, Germany, 1978 

Sl Severins Cathedral 
Cologne, Germany 

Hercules Monument 
Kassel, Germany 

Beverly Minster 
Corbel head 
England, 1978 

God's House Tower voussoirs 
Southampton, England, 1978 

Hampton Court 
Tracery 
England, 1979 

Dorchester Abbey 
Capstones 
England, 1979 

Stratton Methodist Church 
North Devon, England 

Lloyds Bank 

South Molton, England 



Wacker Stone Strengthener H 



Brethane 
Calcareous sandstone 



Brethane 
Sandstone 

Brethane 
Limestone 

Brethane 
Magnesian limestone 

Wacker Stone Strengthener H 
and OH 

Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 



Brethane 
Magnesian limestone 



Brethane 
Limestone 

Brethane 
Siliceous limestone 



Brethane 
Limestone 



Wacker Stone Strengthener 
Sandstone 

Wacker Stone Strengthener 
Sandstone/Brick 



89 



Ramsburg Building Society 
Sherboume, Dorfset, England 

Devon Bank 
Devon, England 

St. Paul's Church 
Munich, Germany, 1980 

Municipal Theatre 
Bern, Switzerland, 1980 

Youth Center 

Hamburgh, Germany, 1980-1981 

Town Hall 

Brugge, Belgium, 1981 

Queen Square House 
Bristol, England, 1981 

St. Michael's Church 
Stuggart, Germany, 1981 

New Castle 

Meersburg, Germany, 1982 

Landesmuseum 

Darmstadh, Germany, 1980-1982 

Neustaedter Church 
Eschwege, Germany, 1982 

Old People's Home 

Effnerpltz, Munich, Germany, 1983 

Orangery 

Weibersheim, Germany, 1983 

Church St. Apostein 
Viemhelm, Germany, 1983 

Kaiser Wilhelm Memorial 

"Gedachtnis Kirche" 

West Berlin, Germany, 1983 

Justice Palace 

Munich, Germany, 1983 

Church "Klosterkirche" 

Ochsenhausen, Obergallgaeu, Germany, 1983 



Wacker Stone Strengthener 

Wacker Stone Strengthener 
Sandstone 

Wacker Stone Strengthener H 

Wacker Stone Strengthener OH 

Wacker Stone Strengthener OH 

Wacker Stone Strengthener OH 

Wacker Stone Strengthener OH 
Sandstone 

Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 



Wacker Stone Strengthener H and 
and OH 

Wacker Stone Strengthener OH 



Wacker Stone Strengthener H 



Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 
and OH 



Wacker Stone Strengthener H 
Limestone 

Wacker Stone Strengthener OH 



90 



Hoechst AG Headquarters Building 
Frankfort, Germany, 1983 

Steeple of Lutherkirche 
FraiJcfort, Germany, 1983 

Tower of Qty Hall 

Charlottenberg, Berlin, Germany, 1983 

Bayer, Vereinsbank 
Augsburg, Germany, 1984 

Alte Pinakothek (Art Gallery) 
Munich, Germany, 1984 



Franciscan Church 
Salzburg, Austria, 1984 



Wacker Stone Strengthener H 
and OH 

Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 
Wacker Stone Strengthener OH 



Wacker Stone Strengthener OH 

andH 
Sandstone/Brick 

Wacker Stone Strengthener OH 

andH 

Conglomerate Stone/Sandstone 



UNITED STATES APPLICATIONS: 



United States Capitol 
West Central Front 
Washington, D.C., 1987 

Indiana State C^itol 
Indianapolis, Indiana 

Severance Hall 
Cleveland, Ohio 

Oberlin College of Fine Arts 
Stone Artifacts 
Oberlin, Ohio 

Soldiers and Sailors Monument 
Indianapolis, Indiana 

St, Paul's Episcopal Church 
Springfield, Massachusetts 



Salt Lake Qty City/County Building 
Salt Lake City, Utah 

War Memorial 
Frankfort, Kentucky 



Conservare H Stone Strengthener 
Sandstone/Limestone 



Conservare H Stone Strengthener 
Limestone 

Conservare OH Stone Strengthener 
Sandstone 

Conservare OH Stone Strengthener 
Sandstone 



Conservare H Stone Strengthener 
Limestone 

Conservare OH Stone Strengthener 
Conservare H40 Water Repellent 
Sandstone 

Conservare OH Stone Strengthener 
Sandstone 

Conservare H Stone Strengthener 
Conservare H40 Water Repellent 
Marble 



91 



MenxMialHall 
Harvard University 
Boston, Massachusetts 

Harvard Medical Quadrangle 
Boston, Massachusetts 



Fist and Second Church 
Boston, Massachusetts 



First Baptist Church 
St. Joseph, Missouri 

New York City Landmark Structure 
956 East 156th St. 
Bronx, New York 

555 Washington Avenue 
St. Louis, Missouri 

New York City Landmark Structure 
770-772 Dawson Street 
South Bronx, New York 

St. Francis Church 
St. Louis, Missouri 

Sl Peter-in-Chains Cathedral 
Cincinnati, Ohio 

City Hall 

Walla Walla, Washington 

Coachman Project 
Clearwater, Florida 

St. Paul's Cathedral Diocesan 

Building and Rectory Building 
National Register and Pittsburgh 
Landmarks Buildings, 1904 and 1926 
Pittsburgh, Pennsylvania 

Roman Temple and Colonnades 
Hearst Castle 
Simeon, California 



Conservare OH and H Stone 

Strcngthener 
Sandstone 

Conservare H Stone Strengthener 
Conservare H40 Water Repellent 
Marble/limestone 

Conservare H Stone Strengthener 
Sandstone/Brownstone/ 
Puddingstone 

Conservare OH Stone Strengthener 
Sandstone 

Conservare OH Stone Strengthener 
Brownstone 



Conservare OH Stone Strengthener 
Sandstone 

Conservare OH Stone Strengthener 
Brownstone 



Conservare H Stone Strengthener 
Limestone 

Conservare OH Stone Strengthener 
Limestone 

Conservare OH Stone Strengthener 
Sandstone 

Conservare OH Stone Strengthener 
Brick 

Conservare H40 Water Repellent 
Limestone 



Conservare H Stone Strengthener 
Conservare H40 Water Repellent 
Marble/Cast Stone 



Limestone Statuary 

Hearst Castle 

San Simeon, Califomia 



Conservare OH and H Stone 

Strengthener 
Limestone 



92 



lOOFTomb 
Lafayette I Cemetery 
New Orleans, LA, 1985-86 

Statue of Religious Liberty 

5th & Market St. 

Philadelphia, Pennsylvania, mid 1986 

Easton Cemetery 

Main Gate 

Easton, Pennsylvania, 1988 



Ohio State Capitol 
Selected Cornice Areas 
Columbus, Ohio 

Trinity Church 

New York, NY, 1990-92 



Conservare H Stone Strengthener 



Conservane OH Stone Strengthener 
Marble 



Conservare H and OH Stone 

Strengthener 
Conservare H40 Water Repellent 
Sandstone 

Conservare OH Stone Strengthener 



Conservare H Stone Strengthener 



93 



APPENDIX D 
MEETINGS ON THE CONSERVATION OF EARTHEN ARCHITECTURE 

o The First International Conference on the Conservation of Mudbrick Monuments 

(Iran/ICOMOS), Yazd, Iran, 25-30 November 1972; 

o The Second International Symposium on the Conservation of Monuments in Mudbrick 

(Iran/ICOMOS), Yazd, Iran, 6-11 March 1976; 

o The Adobe Preservation Working Session (US/ICOMOS-ICCROM), Santa Fe, New 

Mexico, USA, 3-7 October 1977; 

o The Third International Symposium of Mudbrick Preservation (Turkey/ICOMOS/ICOM- 

ICCROM), Ankara, Turkey, 29 September-4 October 1980; 

o International Symposium and Training Workshop on the Conservation of Adobe 

PCCROM/Regional Cultural Heritage Project (UNDPAJNESCO) in Latin America], Lima, 

Cusco, Trujillo, Peru, 10-22 September 1983; 

o Fifth International Meetings of Experts on the Conservation of Earthen Architecture 

qCCROM/CRATerrc-EAG/ICOMOS) Rome, Italy, 22-23 October 1987; 

o Sixth International Conference on the Conservation of Earthen Architecture 

(ICCROM/The Getty Conservation Institute, Museum of New Mexico State 

MonumentsAJS/ICOMOS), Las Cruces, New Mexico, USA, 14-19 October 1990; 

o The Forthcomig Meeting: Seventh International Conference on the Study and 

Conservation of Earthen Architecture (General Directorate for Buildings and National 

Monuments/ICCROM/CRATerre-EAG) will be held in Portugal, 24-29 October 1993. 



94 



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100 



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